One line of thinking is that the crew did end up using the cutout switches at some point, however, by this point it had been trimmed nose down (due AoA fault operating the MCAS). Now the electric trim is unavailable with significant nose down trim, the only option left is manual trim (very slow) or to reduce IAS the air-load Speed increased to the point where the elevator could not overcome the trim.
Given the undocumented MCAS system and an unannunciated AoA fault, whilst dealing with an unreliable airspeed, it is highly unlikely that many pilots would have the insight or knowledge to understand the need to reduce speed (and how do you know what to set, the only guidance in the checklist is two attitude / thrust combinations depending upon the flap configuration), or recover with manual pitch trim from 5000'.
A very interesting simulator experiment would be to do a series of progressively more nose down trims, setting the UA QRH attitude & thrust and seeing if there is a speed or nose trim position that becomes unrecoverable using the elevator alone with the stab trim switches in cutout.
The more official and semi-official documentation and information that is starting to come out (ie the APA MCAS) suggests it's not looking good for Boeing.