Haha, dude saying that someone is a blackbelt doesn't have much to do with anything because Jiu Jitsu has any different styles and applications. So, saying that Brandao is a blackbelt over and over doesn't really emphasize a ton considering he's rarely showed any great takedowns or top pressure/control, but quite the opposite: his opponents tend to fairly quickly generate scrambles from the bottom when he is on top.
It's like saying that Kenny Florian couldn't take Gray Maynard down and submit him as if it's some big deal, because Kenny is a black belt and Gray is a blue belt. Completely different styles of grappling, very odd thing to brag about.
Jon Jones has fought many wrestlers, the best wrestlers probably on any resume currently in the UFC in DC, Bader, Hammill, Rampage, Rashad, Matushenko and so on...he's also submitted black belts like Machida and Belfort...he's not a good analogy for this.
The point is, Conor has yet to fight someone who regularly and expertly takes dudes nu down and grinds on or submits them from the top. Period. He's also been submitted twice in pretty embarrassing fashion from people who took him down and got top control. With these variables in mind....it's a fair assumption that this might be a weak spot. It might not be, as well, but there's certainly more evidence that it is than there is that it isn't.