Since then? The Pe De Pano fight was like 2 months ago, I was there for that one. Sylvia is bigger, meaning if he wants to (which he usually does) use a jab, he's gonna give Monson even more problems unless he improved light years since that fight. And the fact that knowing Sylvia is a striker, and not only that but the champion, Monson will likely be more tenative on the feet than he was with Pe De Pano, a fellow submission guy completely new to MMA that was out-striking him.
And about owning Sylvia on the ground, two other points you didn't make was not only will Monson have trouble getting the fight there, but getting it there and being on top, and keeping it there, are totally different things. I say that because the only way Monson "owns" Sylvia is on top, as submission wizard or not, underneath Sylvia's giant ass is nowhere he wants to be.