The diabolic xill would probably be allowed under Bardic Knowledge. Just because the beginning says that "bardic lore isn't going to help much" doesn't mean that you can't find out anything from bardic lore, but seriously, how much would it help you to know the xill's backstory after you kill him? Not much (It would be pretty cool to think up appropriate details though, and to hear the story in a roleplaying perspective, so that the enemy becomes more than a nameless foe).
That said, I agree that eliminating all possible bardic knowledge checks if that other guy was able to research it is very poor design. However, that doesn't change the fact that your original argument was specious. If you had started with:
"In World's Largest Dungeon Area A, there is a wizard who has researched the dungeon with the aid of his familiar, and yet they dismiss the possible of a bard knowing anything from bardic knowledge offhand. This is a poor design choice that hurts the bard. "
I probably would have agreed that this was poor design. It is important to use the right tool for the right job. As someone who understood all your references, I would assume that the first rant was aimed mainly at people like me, since anyone else would just scroll through it without realising their import, so your choice clearly wasn't aimed at them. And even so, I don't buy the original argument.