Ditto, kind of. I did 4 months of HFLC + Z2/low HR training, like the OP, for health reasons. The first couple of weeks were a bit sketchy as the rides got longer (3+hrs) but didn’t take long to be able to do carb-free/fat-fuelled back-to-back 5hr jaunts.
But yeah, unfortunately repeatedly smashing out Z5+ intervals successfully does require carbs.