Because his profession to this day is Sculptor. why else would they hire him? Just because he only started work in early July doesn't mean they started filming any scenes that required what it is they hired him to sculpt. they shoot movies out of sequence and they have only been filming for the past 8 weeks... Heck, they are still hiring actors with 3 just hired only this past week!
while we can speculate on the technology of how they are making the costumes/ armor/ helmets and granted it looks like Kevin Smith saw stormtroopers a few days prior to Brian Muir's announcement we don't know that he wasn't hired to sculpt a different helmet/ armor that has yet to be filmed. Yes, I am speculating but being JJ is going for real sets, props and costumes and doing much in the old school traditional sense and hiring many of the OT staff I feel it would only make sense you would hire a sculptor to
sculpt something yes?