Simple answer is that the date is wrong; even if SA-kuva.fi is the official Finnish military image archives, its catalog is not devoid of errors. That is, if that year comes from the catalog entry; I wasn't able to locate this photo in the archive so I don't know what it says.
I did however locate two other photos that are probably off the same film roll, numbers: 56495 and 56496. (Using Imgur links here since the site doesn't seem to have a function to link directly to the catalog entry. Putting those image numbers in the site's search box works though)
Both of these other photos are labeled as October 1, 1941. Which makes good sense; Petrozavodsk (Äänislinna to the Finns) had just been taken in September of that year So we have an international group surveying recent Finnish gains, and America is not yet at war with Japan. He wasn't the only one; there are pictures of Swedish prince Gustaf Adolf (VI as king) touring the area around the same time, although he's not in this photo. (The man holding a camera looks like he's wearing a Swedish m/1939 uniform though)
You can't trust common sense though. (In fact I'd say the biggest source of mistaken beliefs is due to not scrutinizing things that appear to make sense) So looking further we have the fact that they're standing in front of the Lenin statue, which the Finns removed. Photo 64886 dated November 21 shows the work in progress. A photo 80782 from April 1942 shows a cannon on the spot, and another photo of it (82640) has the caption (translated): "The Soviets had erected a gigantic Lenin statue on this stone base, which our troops changed to a cannon taken as a war trophy".
So it isn't just unlikely that an American officer would pose with a Japanese one after Pearl Harbor, this picture could not have been taken at any other time than from mid-September to mid-November 1941. After the Finnish offensive but before the statue was removed. The October 1941 date is correct.
https://www.reddit.com/r/MilitaryPorn/comments/67r0kr/finnish_german_italian_french_american_and/
Most of the officers you think are german are finish. Beside the finish you see plenty of military observers from different countries (Vichy France, Italy, Sweden, USA, Japan, Germany) The official stamp is 2 october 1942. The stamp is obviously incorrect. (the Lenin statue wouldn't ever survive a full year in finish city back then.)
If to check the full set of the relevant photos on http://sa-kuva.fi stamped with october 1942 you see plenty of photos with the description like:
" Ensimmäistä suomalaista apteekkia perustetaan Äänislinnaan. Lokakuu 1941.Äänislinna 1942.10.06 "
Locakuu is october.i..e these photos are made in october 1941 and dated in archive by october 1942.
