Simply acting on a flight control cannot exclude the other.
OK, but you also say that an Airbus TRE must only correct a situation by pressing the override button - which does exclude the opposite side-stick - and "I have control".
Yes, of course training must be clear and understood, but completely taking control from someone (instead of gently assisting), is a big deal and puts the cadet on the back foot, thinking they have done something bad. That Captain PM at JFK who dramatically emergency stopped rather than simply gently correcting my taxiing caused me a lot of concern and alarm and put me right off at a time when we both needed to be focussed and very aware of ATC and everything going on at that airport before we took off.
You might be right, but when someone has the wrong picture of the flare in their mind, a gentle correction there-and-then by a TRE might sort it, rather than discussing it in the classroom afterwards, when the picture is not there in front of you.