I can't comment on Disney as I have no in-depth knowledge of the man. But pending a longer answer on Disney I'd like (mod's allowing) to just set straight a few points about Riefenstahl. That poster is clearly misinformed.

Riefenstahl was never a member of the Nazi party or any party organization, either in 1938 or later. Her films do not contain any explicit racism or antisemitism.

That said, her sympathies for the party are much-debated. What is not, is her personal loyalty to Hitler. Besides her debut Blue Lights (1932), all her films by in 1939 - the works that made her famous - had been Nazi propaganda vehicles. (Der Sieg des Glaubens - 'Victory of the Faith', 1933; the infamous Triumph des Willens - 'Triumph of the Will', 1935; and the two parter 'Olympia' in 1938)

Whether 'Olympia' ought to be considered Nazi propaganda is however a quite debated and contentious issue. The film does not explicitly advance Nazi ideology. On the other hand it's a film financed and promoted by the Reich Propaganda Ministry. Between those poles you have subtle issues like what actually constitutes propaganda. A lot of ink has been spilled in film studies over it, but fortunately for the sake of brief post, that debate is irrelevant here.

What's relevant is that, right or wrong, the film was widely perceived in America to be a Nazi propaganda film. It was boycotted and did not gain distribution.

Riefenstahl arrived in California on November 24, 1938. The Hollywood Anti-Nazi League ran advertisements against her in the Hollywood Reporter and Daily Variety on November 29th 1938. Stating "There is no room in Hollywood for Leni Riefenstahl!", and calling her the "Head of the NAZI FILM INDUSTRY" [Capitals in original] - which is certainly an exaggeration.

Allegedly, she'd wanted to meet with Louis B. Mayer, who was interested but, due to her reputation did not want to meet her at MGM, leading her to angrily refuse, and tell the press. (Leni Riefenstahl still getting business' brushoff, Variety, December 7th) At the same time, there were those defending the innocence of the film she was there to promote. For instance, in pieces in the Hollywood Citizen-News (December 16 1938) and the next day in the Los Angeles Times defended Olympia against Nazi allegations and praised its artistic merits.

It was in the midst of this; on December 8th, that her Disney tour took place. Her friend, the artist Hubert Stowitts accompanied her. He had been the one to arrange the visit, through his friend, the conductor Leopold Stokowski who was working on (what'd become) Fantasia at the time.

Was Walt Disney aware of who she was? According to her account, he certainly was. According to her memoirs (published in 1987), he knew of the film since both it and Snow White had been nominated at the 6th Venice Film Festival in August that year, and wanted to see both parts of it. (implying he'd not seen it though, or at least not in full)

She said she had copies at the hotel that could be brought for him for a private screening, but he declined, saying Hollywood would find out and it'd ruin his reputation. He claimed his projectionists were unionized and since he didn't have his own theaters and distribution, he couldn't afford them boycotting him.

Riefenstahl did not claim Disney offered to work with her, or that she asked to work with him. Above all she was trying to find distribution for Olympia in America, which was a doomed prospect. The Kristallnacht had occurred while she was in America (November 10th) and this had naturally exacerbated anti-Nazi sentiment in America. Riefenstahl, who as said, believed american media to be controlled by 'anti-Nazi forces' (see below) had little problem dismissing/diminishing the events as reported. Which is why she'd stayed in America to go on to Hollywood despite advice to go home. If Disney wasn't going to screen it privately, he certainly wasn't going to distribute it.

Nazi-controlled media in Germany had long perpetuated the claim that Jews controlled Hollywood by this point, so when Riefenstahl failed to achieve a distribution deal or much of anything else in her America trip, there was no question of who was to blame. Berliner Illustrierte (19 January 1939) ran a photograph of Riefenstahl in country attire with a horse, with the caption "Leni Riefenstahl on a visit to a Californian farm. Jewish influence in Hollywood is preventing any public screening of the Olympia film, although Leni Riefenstahl received enthusiastic applause when it was shown to film critics."

Riefenstahl herself had made her statements more subtly, having explained to Film-Kurir (No 2, Jan 3 1939) : "The American film industry, including production and distribution, is controlled by people hostile to modern Germany. Again in Film-Kurir (No 27, 1 Feb 1939) she also hinted that fear of an anti-Nazi cabal that ran Hollywood had hampered her: "I cannot tell you which other directors or actors I saw [other than Disney], as it would cost them their jobs. I had to promise them faithfully not to 'betray' them."

This is antisemitism with "plausible deniability"; Riefenstahl never said 'the Jews control the media', but the reader was clearly able to make such an inference. And the German public had been primed to do just that for half a decade at that point.

In her memoirs, from the postwar era, Riefenstahl's terminology is different, laying the blame specifically on the Hollywood Anti-Nazi League: "I could see three months later how powerful the Anti-Nazi League was from the American press reports.Walt Disney was forced to make a statement that he didn't know who I was when I visited".

Both accounts could be correct though; it's possible Disney may not have known when he agreed to the tour. But as said, she was famous for her films by this point. She'd been on the cover of Time Magazine in 1936 (with the caption "Hitler's Leni Riefenstahl"). There's ample evidence that she and her visit were well-known and well-publicized at the time. (Whether Walt was a Variety reader, though, I do not know)

Riefenstahl is a tricky figure. She aided the Nazi party but was not a member. She apparently believed in and promoted anti-Semitic narratives (as in the example above). Yet there are few (although not zero) overtly, explicitly anti-Semitic acts and statements attributed to her. She herself always denied being an anti-Semite or Nazi sympathizer, for what that's worth.

Although that may not be worth much, Riefenstahl was no Josef Goebbels or Julius Streicher, who publicly denounced Jews on a daily basis. She held forth - even at the time - that she was independent, not a Propaganda Ministry employee, not a party member, etc. She was still not the 'leader of the Nazi film industry' that the Hollywood Anti-Nazism League had painted her as.

So while it might be unlikely Walt Disney would be unaware of Riefenstahl's reputation, there's also a case to be made that he did not believe she was an antisemite. As said, some American film critics were defending Olympiad (and indirectly Riefenstahl) from charges of antisemitism and propaganda. To see her antisemitism you have to look at her acts, rather than her words, and the bigger context.

But the critics were right; the film is to this day considered an artistically significant film despite its director's abhorrent views.

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Sources:

Leni Riefenstahl, Memoiren, 1902-1945, 2 aufl., Zeitgeschichte, 1994

Cooper C. Graham, ‘Olympia’ in America, 1938: Leni Riefenstahl, Hollywood, and the Kristallnacht, Historical Journal of Film, Radio and Television, vol. 13, No. 4, 1993

Ranier Rother - Riefenstahl - the Seduction of Genius, Continuum, 2002