There is a reason Fire Blast, and mixed sets in general, are popular on MGarchomp. Stone Edge doesn't let you kill Ferrothorn, Celesteela, Skarmory, Scizor, etc. and Fire Blast happens to be the most lethal attack in Garchomp's movepool for those Pokemon. Many of those same Pokemon check Excadrill, so Fire Blast is even more relevant on this team. Then, in addition to that, Outrage is an inferior attack to Draco in a meta where Fairy-types and Intimidate are so prevalent. There are real incentives to not go fully physical.
You can run an SD set as well -- I recommend dropping Dragon coverage entirely, in that case -- but it isn't superior to the mixed set in every situation and requires a bit more justification that it's being given here.