I just don't see them applying the damage reduction to FnB spells, or else you wouldn't be able to take that talent on fights with any aoe whatsoever. You'd be swimming in embers much faster than you could spend (one FnB incinerate on 3 targets would be a net positive on embers -- with 4 targets and a decent crit rate, you'll be at 4 full embers in two or three casts) ... and if you don't have a way to spend them it's pointless
I guess it comes down to specific scenarios. 'Cause on a fight where there are two or three adds and the boss is in execute range, your rotation turns into "FnB Incinerate x1 -> Shadowburn, rinse and repeat"