A worn bearing in the starter itself will howl like that. It's just a bronze bushing, and as it wears the clearance between it and the shaft gets larger and the shaft, instead of staying centered and spinning freely in it, starts "walking" around the inside of the bearing, making that noise and reducing the RPM of the starter motor. I don't see any evidence of a slipping starter adapter clutch at all.
The mechanic could take the starter off the adapter and connect it to a battery using jumper cables. I bet it will make that noise.
It's worse in cold weather as the lube in the bearing stiffens.