MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall.
(my emphasis) I'm reading this to mean that the intention is to "assist"(TM) during steep turns and not that a steep turn is a condition of MCAS operation.
That said, if the ATC is correct, there were issues including unusual speed/acceleration for most of the flight which would possibly have been with autopilot engaged when MCAS isn't active. The pilot requests turnback and then disappears off radar.
So, accepting it contradicts Occam's Razor and needs another cheesy hole, perhaps something else non-MCAS-related was seriously wrong, but then on turning back, MCAS nose down trim becomes the final nail in the coffin as evidenced by trim jack screw position on ground.