Less sorcerer for more warlock in D4’s Holy Nova build?

https://m.youtube.com/watch?v=9Er-sUm2b6E&ra=m

So D4 recently did a build that utilized Paladin, Warlock and Sorcerer levels, building around searing and eldritch smite. The build shown is a 1/9/10 split, but I wanted to do a 1/12/7 build to get devouring blade for three attacks instead of two. This looses 2 5th level spell slots and 3 level 5 spells and a level 4 spell, but also gains an extra level 5 pact magic slot. That being said, I’m only suppose to be using those slots for smiting. 

So, my question is do I loose that much from draconic sorcerer by dropping 3 levels in it in exchange for 3 in warlock? If not, why do you suppose D4 chose to do it the way he did? Hopefully this was an ok place to post this; I thought it had the most to do with the sorcerer class.