deck Whitman said:
How much does writing songs on the guitar weigh compared to technical brilliance?
It is arguable that Dylan was a great guitarist, for example. Not because he was technically good. He strummed chords. But because he used it as a vehicle through which to write brilliant songs. (Same with Springsteen, for example, although he is technically stronger. And probably a huge reason Richards is so high on these lists - although he is technically outstanding, as well.)
You are right, composing must be part of the criteria for inclusion on the RS list otherwise some of these rankings are crazy.
I'm familiar with basically all of the 'classic rock' players listed here and a surprising amount of their output, in some cases, is written on the piano and fleshed out on the guitar, Lennon and McCartney being the most obvious examples.
My take on Harrison is that he was a master at all those delicious little 'fills' he played on so many classic Beatles tracks and that he was criminally under rated as a player especially and composer. He's no McCartney as a writer but a better player, for sure.
Another glaring ommission from the Rolling Stone list is Martin Barre (Jethro Tull), who has chops rivaling most in the top 30.