They deactivated the trim after allowing MCAS to put the aircraft so far out of trim as to make it almost uncontrollable. Allowing the airspeed to exceed Vmo while the aircraft was out of trim is what eventually made the aircraft uncontrollable.
If you read the transcript it shows that the first two MCAS activations resulted in some 5 units of nose-down trim movement. The pilot used the yoke-mounted trim switch to move the trim approximately 2 units nose-up, but then MCAS reactivated and trimmed back in the nose-down direction. THAT’S when the crew deactivated the electric trim.
What they SHOULD have done (per the emergency AD they were purportedly award of) is use the electric trim switch on the control yoke to get the aircraft in a trimmed condition, THEN deactivate the electric trim. The way the system was set up, activating the trim switches on the yoke deactivated MCAS for five seconds after the trim switch was released, which gave the crew more than enough time to deactivate the trim after manually trimming using the switch, and before MCAS reactivated. All of this was covered in the emergency AD, but pointing that out has somehow become “victim blaming” or something…