800 doesn't have MCAS.Actually I think this is MCAS failure again. Looking at it a bit more closely the altitude and profile of the descent is consistent with it. MCAS would have detected a stall and put it into a dive which is roughly 20s long. It’s possible at 3,000ft the aircraft was reporting an incorrect speed or the pilot had erroneously not put the flaps down yet and had actually put the aircraft into a stall. Flaps down would have disabled MCAS.
edit: posted a different view below.
Got Boeing on site Wednesday I'll quiz him see if he has any info he can share.