You lodged a claim under EC261
I believethat it has been clarified that the arrival time for determination of delay is when the doors are opened - as per BA's response
It has indicated ( I asusme correctly ) that this was within a 3 hour delay

If these are true what is the issue with the response?

it seems to be factual and addressing the claim

the reason for delay in this instance is irrelevant if the delay was not long enough

If you had just written in to complain about having to sit on an aeroplane for so long rather than claim for compensation , the response may have been different.