Can confirm, I have a 2017 Prius Prime, I keep it set to prefer all-EV driving, but I just got in and turned on the defroster to melt some frost off the windshield, it switches on the ICE engine. My only complaint is that it is not terribly smart about turning it back off during the same session of driving, if I turn off the defrost. I think in theory the heat pump would be able to provide the heat to do the job, but it'd use the charge in the traction battery up quickly...