That's the problem. Tyler went to turn off the SYD and turned off the EFAS (which wasn't active anyways) by accident. People are taught that the SYD is there to help you, which it is, however the aircraft flies just fine without the SYD on. The training aspect, or lack there-of, instills into pilots in PIQ to only kill the rudder power if you have unscheduled rudder deflection, not rudder oscillations, SYD issues, etc. Ever lose lose the right hydro system? You're going to lose the SYD as well. I've lost the SYD a handful of times flying and is it a pain with all the aileron inputs for the PF? Sure. Is it manageable until you land, absolutely.
They should've slowed down, however the AIB's report states Tyler was putting rudder input while trying to level off and turn to the fix they were going to hold at, which the -1 says not to use any rudder inputs to counteractive dutch roll, which even had they slowed down we'll never know if that would of lessen the stress on the tail enough not to have it fail.
The jet had SYD/PCU problems, started to dutch roll, the crew misdiagnosed the rudder problem because they hadn't been trained to analyze the type of rudder malfunction they had, and Tyler put in rudder inputs while the aircraft is in a extreme dutch roll, which made it worse and eventually the tail to separate.