The important thing to keep in mind here is that Buffett is talking about "saying no to almost everything" when you are already successful. Not when you're actually still in the process of becoming successful.
The rules are different there, although of course Jobs' quote about focus, I think, applies to both situations, to some extent.
He does not just "no", he know his time is finite. As long you have (free) time, then yes, you might say yes to something more. But as more you climb the ladder up, the more people come to you with all kind of things. And then even if you would like, there is not enought time and energy in a person to do everything for everybody. I guess, thats a reason why people burn out. Even when people work so hard, if you say yes to everything, there is no chance ever to finish your many many tasks. Learning to say "no" is very important.
The rules are different there, although of course Jobs' quote about focus, I think, applies to both situations, to some extent.
EDIT: Grammar.