This. DC got hurt by every big shot he ever took, as well as had an additional big weakness to body kicks and leg kicks. His ability to absorb damage isn't anything to write home about.
Like you said, it's the fact that if you're not putting him out cold / completely compromised you have to get in close to him and then his grappling instincts kick in and he ties you up. That has nothing to do with his actual chin / damage absorption ability. Mir, Rumble, Gus, Silva, and Stipe 3 might've all also finished DC if he didn't tie them up to prevent it.
Which is the issue that you've always had to deal with with great grapplers. Guys like Fedor, Nog, Hendo, Werdum, etc all had the advantage that even if you hurt them / knocked them down they were still in their world of expertise and muscle memory is real. It's a hell of a lot easier to blindly grab onto your opponent than it is to counter them.