federer did use the slice dtl but the slice to nadal's fh will not work with regularity.
the dtl slice is not easy to execute and has to be used only when the ball is in the right spot in the court.
Yeah, Roger should really be honing the DTL slice: in practice and match play. He obviously feels better going CC with it.
Also: I'd have to rewatch the match, but Rafa could've adapted and made the DTL slice an even lower percentage shot. Or a shot Federer felt less and less comfortable hitting.
Anyway, there are a lot of folks who describe Federer as being "stubborn" when not executing the obvious game plan against Nadal. It's just very difficult to execute a plan you've come up with just for one player, a great one at that, and on the biggest stages of the sport, in front of folks and while feeling serious pressure. Perfect conditions for a series of brain cramps. Especially when you have too many different shots at your disposal.