1. Is it possible he was making fun of his dad? Writing a whole book as a joke might seem unlikely, but this was a guy who as we know was a huge fan of the 'false documents' technique (it's all through Tekumel). Doesn't seem out of character.
2. This was in 1991. Is there any reason to assume he maintained Neo-Nazi beliefs for the next 20 years? (He died in 2012.) Did he ever advocate for them in any fashion over the next 30 years? Are we going to judge a person by every dumb thing they've ever done? He made one of the first RPGs to use non-European inspirations, which is somewhat odd for a white supremacist to do, particularly in the late 20th century. While I am aware of the Nazi occultist current and attempts at synthesizing Islamic and Nazi philosophy (not to mention Hindu-Nazi authors like Savitri Devi), the one thing I seem to get from reading his life is that he was an extreme xenophile (and maybe this was a rebellion against Dad).
3. Again, this was in 1991. White nationalism was much weaker then than it is now, and he might not have felt as bad (assuming in fact he was not a Nazi) about writing a fake white nationalist book if it wasn't going to convert anyone. A vaccine conspiracy plot is different in 2000 (remember the X-files smallpox vaccination subplot?) than in 2020.
4. Yet again...1991. There was much more of an acceptance of 'edgy' humor back then, and it might have been something along the lines of 'let me write this totally crazy thing to make fun of my dad'. (I think my attitudes are probably closer to 1991 than 2022 TBH).
I am half Jewish by ancestry (and lost a few extended family members to the Nazis), but am not inclined to 'cancel' Barker over an obscure book he wrote in 1991. Indeed, I think I will go and donate a little to the Tekumel Foundation. We used to make stuff like The Producers, now we're digging up 30-year-old dirt on game designers who have been dead for a decade.
Now if you want to talk about Louis-Ferdinand Celine...