This final boss fight i would actually say is really memorable. The game has that certain spark to it and very memorable bosses, like the first miku, 2 fangames i know of even followed down its path, which were I Wanna Be The Lunatic and I Wanna Be The WB, but on to the point.
Big kid is possible, we know this, people beat it, but Extra Big Kid, Nekoron claims he beat it, and he does not have proof of it, I don't know Nekoron so should I believe him? But why its impossible is because of 5 bound cherries flying around, the cherry speed is increased, and the bullet speed is increased, it's way too much to handle. The TAS of it kinda proves how hard it is right toward the end of the fight, or is it possible that this fight was meant as a little trial thing where you beat your high score.
I think the best way to find out more is to ask someone who has been in the Japanese community since Charr was released since Nekoron was still around i think, Something to do for yourself though, I don't really have many connections there.