Beside he is clear that his transmitter did shut off due a low battery, the runaway has everyting to do with that.

Transmitter shuts off -> no signal -> failsafe pops in -> because it was not configured to the car it took whatever it had programmed last time (probably from another car)

But still you do not see that issue.
And yes, in his case it is possible to shut off the failsafe in the menu of the 4PX transmitter but but probably did forget it as it is something you normally do not care about. Again, not with all transmitters you can shut off the failsafe so it is something to be aware of.