Mitnick has been denied access to a computer for years. He has kept up with some reading, but has very little practical experience on current systems. As such, he probably farmed out the sysadmin job (in which case cracking the site doesn’t really make the intended statement) or he did it himself as a relative newbie. Besides, Mitnick didn’t gain his fame as a web cracker. He was into phone systems and general Unix cracking certainly, but there’s nothing in his past that indicates he’d have any specific skill as a website admin.
Since he’s one of the most famous crackers, it’s completely expected that he’s a target for everyine trying to make their bones. No site, even if Mitnick were up to date, is uncrackable, so the fact that he’s made himself a target makes this inevitable.