Look, IRL you cannot convince someone of just anything, and it's not a matter of how convincing you are.
If you wanted to convince me that I am, dunno, Soro, it would take much more than a crazy high bluff check. It would take elaborate planification, weeks if not months of those crazy high bluff checks. It would take acting, putting me in some kind of truman show.
To me, the point of glibness is that you can convince anyone that you are absolutely honest in what you say. That doesn't mean the BBEG is gonna trust you if you told him he's good, he would just believe you are really convinced of that.
In a world with magic, that's strong and possibly unbalanced, but it's concievable. There are detect thoughts and similar, as a counterpart to crazy high bluff bonuses.