Yes, that's all true, but still irrelevant. I understand that all three are individual AIs, and could be made into AI nations, but normally they are not. (It's a neat idea and I can see how it could be fun to play with.)
In normal usage AI nation refers to the same nations that a human player could play but controlled by the computer, not to random attacks that are grouped together for convenience into things that could be nations but aren't.
So when someone asks, "will the AI attack unconnected provinces?", to reply as you did may be technically true, but it's at best misleading.
It's also misleading to say that
random attack events sometimes reinforce independents. As far as I can tell, barring map or mod hacks, it's not sometimes but always. "Almost always" would have worked, as would "Always, with the following exceptions ..."
"Sometimes" is just wrong.