Why is he so obsessed about telling people that he's only a songwriter nowadays.
Because people, journalists in particular, keep overlooking that (maybe most significant) aspect of his talent. Everybody keeps calling him the "great guitarist". There's nothing wrong with that; he is and certainly was a fantastic guitarist. But it must be annoying if people keep reducing you to this when you're actually one of the best songwriters in the history of songwriting.
And about the Nobel Prize: I think it's generally questionable whether a songwriter will qualify for that award, but at least
today, Mark is a much better lyricist than Dylan is. Don't get me wrong, I'm a huge Dylan fan (also of his late work), but in terms of lyrics, he's really just pasting together whatever he finds in the books he reads. That's okay when it works (and it often does), but hardly Nobel-Prize-worthy. Whereas Mark deserves his Ivor Novello award more than ever.