Yes, declaring that anyone who doesn't believe that Yxa is the answer is suffering from cognitive bias is definitely presumptuous. That statement is absolutely making assumptions, and using that to cast unwarranted negative aspersions on anyone who disagrees with you.
And you just did it again by referring to UFOs and homeopathy. This is not a case of conspiracy, this is a case of people looking for possibilities. By making analogies to truthers, and painting a conspiracy theory in advance, you are undermining Dan Bron's work and demonstrating your own cognitive bias.
You're choosing which proof to believe. The image of the (presumed) axe on the ball is yellow, that's also "substantial proof". It is, in fact, yellow. It's not unreasonable to suggest that there's better direct evidence, and a simpler explanation, for the "yellow" answer than there is for the "Yxa" answer.
Maybe the designer was Asian and accidentally saw the Y as an upside-down A, maybe "axe" was the word they were going for. Maybe the designer was Asian and used yuè, the pinyin word for axe. (https://english.stackexchange.com/a/395521)
It's not likely that a definitive answer will be found, so let's enjoy reading and speculating about it, bask in the awesomeness of Dan Bron's narrative and presentation, and lay off the unjustifiable holier-than-thou attitude that there's only one possible answer.