In my current save as Scandinavia, I have yet to notice the AI support rebels (at least in my country).
Yeah, I don't see them do it.. they do send spy's to make claims against my country but I don't see any foreign funded rebels popping up that are not funded by me..
Likewise.. Aragon isn't supporting rebels in my country, but hell they are making claims like there's no tomorrow
AI does not currently use support rebels. It's on my todo.