As for "mastering" a song, I get what's being said both ways.
I think having mastered a song means you truly copped the way its played- all the little nuances of his/her technique, any sloppiness in their timing, their vibrato..., all of it. And of course, the notes. And unless you've already spent a ton of time learning to play like that particular guitarist, learning one of their songs so well that if you played it through their rig, no one could tell the difference it wasn't them, I do agree it would take a lot longer than a handful of hours.
Paul Gilbert whipped out VH
Spanish Fly on a request, and even though it was damn close, he could probably spend less than a couple hours more, and nail it exactly. But he'd already put in the work to be able to play at that level.
So, say for example, if you're an alt-picking shredder, and someone challenges you to learn a difficult song in a finger-picking style of Chet Adkins or Tommy Emmanuel, you ain't gonna get it in mere hours, because you'd have to first spend a lot of time learning that technique.
I think. Unless perhaps you're one of those autistic savants, or just so naturally gifted that it makes people sick.
But if you're already fluent in a particular style, yeah I can see a person really
nailing a cover song in just 6, or whatever, hours.