There is a kernel of truth to this claim, but times they're a-changing.

The Finnish Civil War, while brief, was a very bloody and traumatic event for us Finns, and for decades one could tell a person's family history from the word choices s/he used about the war: those from White families talked about "war of independence" or "Red rebellion," while those from Red families talked about "rebellion," "insurrection," or even "class war." The two sides had very different narratives about what had actually been the causes of the war, how the war had been conducted, and, most of all, who was to blame.

However, until the end of the Second World War, the Red narrative was basically proscribed, and "official" Finland celebrated the White heroes who had saved the country and/or made it independent against Red Russians and their misguided Finnish henchmen. This echoed the wartime White propaganda that depicted the fight to be against Russians first and traitors second, even though Russian involvement in the war was limited to some volunteers (mostly from Imperial Army units that had been stationed in Finland) and some equipment. It is fairly universally accepted these days that this propaganda was believed necessary to induce conscripted smallholders and tenant farmers from the White-controlled northern and western Finland to fight against the Reds, who were from similar socioeconomic strata. This was the official line taught in schools after the Civil War and to some extent up until 1970s (or even further, depending on the teacher), and there is no doubt that a major reason was to brush the painful wounds of the Civil War under the proverbial rug. Before and even after the Second World War, having a family background in the Reds could be by itself sufficient reason for a conscript to be denied admission to the Reserve Officer School (a prerequisite for a military career as well) and even to some places of employment.

After the Second World War, the Red narrative emerged gradually to public discourse. A milestone was the 1959-62 publication of Väinö Linna's Täällä Pohjantähden alla (Under the North Star [Polaris]), a three-volume epic about life in a semi-fictional small Southern Finnish agrarian village from about 1885 to 1950. This book, perhaps more than any other, brought the Red narrative to the open: Linna, a social democrat who had risen to fame by his previous book Unknown Soldier, the most controversial and the best novel about the Continuation War (and, IMO, one of the best war novels ever - but in original Finnish only, unfortunately), very clearly favored the Reds in his book, even though his deeply ironical touch makes them, too, seem both tragic and flawed figures not that different from the Whites except in terms of wealth. Historians think Linna's portrayal of the events was in general fairly accurate, a layman's social history so to speak. In the last volume of Under the North Star, Linna also fairly accurately - if very ironically - illustrates how the Winter War brought the former Whites and Reds together, with sort-of apologies issued from the Whites and mass graves of executed Reds being consecrated as graveyards, for example, while many former Reds express their anger against the Soviet Union, now seen as the outside aggressor. (This, too, is firmly based in reality, although issues were always complex: nevertheless, after the outbreak of the Winter War, quite a few of former Red soldiers reported as volunteers despite their lack of formal military training.)

From 1960s onwards, the post-war generation's general denouncement of their father's values swung the pendulum to another extreme, and the Reds were now celebrated as the vanguard of the proletariat and heroic defenders of the downtrodden. This hero-worship was mostly limited to student circles, though, and abated with the general disillusionment with leftism and communism that began to take root from the late 1970s.

These days, close to 100 years after the Civil War, the wounds do still exist: right now, a populist Right politician is under fire because he published an album where he sings about "Red whores" of 1918. However, outside the ranks of loony Right and Left, the bygones are bygones. Many Finns are aware of which side their ancestors were during the war, but it doesn't cause any issues any longer. There is now much more research about what actually happened, and much of it is critical of the "official line" (which hasn't been the government's stance for decades, though). As for difficulties in accessing archives, I have to say I haven't heard of any problems, but I wouldn't be surprised if some materiel was unavailable until fairly lately.