Self deprecation is a very British thing to do. Thats all it is I would say.
The thing is, you can't just see Mark say 'I'm a teacher's worst nightmare' and assume he's a modest guy. He just thinks deeper than that.
I can play something, of course, but there's literally millions of guitar players better than me out there, and this is just a honest look.
People often say to me things like 'you play good, man, wow, you play so good' or my favourite 'I wish I could play like you'. I LOL at it.
Self-deprecation plays its role here, but not so much. In case of Mark, his overall sound obscure all his technical capabilities.
For example, I make a lesson on how to play his part in 'I'll See You In My Dreams' 1987 version. And what I can say? Well...
It almost impossible. Literally the teacher's worst nightmare, because there's so much information. I can play it, but explain? Er...
In other hand, the lesson for Chet's part took me one weekend to pull out, because it's a straightforward virtuosity right there. On or off.
Same goes with Mark's songwriting. Many people will say like 'Mark is the best songwriter on the planet', but you know this is exaggerated.
And he knows this is exaggerated. There are thousands of good songwriters out there and dozens of almost-genius songwriters.
So Mark is not so modest so to speak, but rather have a very intelligent approach on explaining his own skills.
I'm Jack of all trades, master of none.