Did the pilots just not look at the altimeter? I’m not a pilot but it seems to me once the autopilot is set you’d naturally look at the altimeter and maybe airspeed indicator to confirm it was working.
Altimeter is a secondary instrument for a climb. The primary instrument for pitch would be the airspeed indicator (for a typical constant speed climb). The fact that they noticed the descent (on the VSI and altimeter) and corrected indicates they were scanning, but doesn't tell much about what was happening that caused them to transition from a climb to descent.
Yeah, agreed here - workload on a departure (depending on the type of departure) can be pretty heavy compared to cruise phase. Arrivals and approaches are more commonly known to have heavy workloads, but there’s a lot to do just after departure, so this doesn’t feel wildly out of bounds for a scan recovery after an autopilot mistake.