Asserting he used a key does not lend credence to abduction, it messes it up.
Asserting Kate used the key to get in was a further mess. To use a key she would have to have passed the open shutter. Why he said anything about Kate using a key when he did not go with her is very odd.
The impact was that the first statements of the Tapas 9 appeared to have major problems, which did not advance the abduction theory.
As to why he did it, I can give good 'reason', but that would be speculation, which I understand is not welcomed here.
If it was a misunderstanding about which door was used (e.g., confusing "back" and "front"), the officer could easily have assumed that if he'd said (or the "interpreter" had interpreted) that Kate had used the same door, then it must have been the door requiring a key.
Wouldn't a police officer normally attempt to get the gist of the layout of the apartment? If he did, it's not in the statement. If he did, then it may well be part of an informal discussion prior to typing up notes. If he didn't ask... why didn't it occur to him?
There is no evidence of any discussion concerning the layout of that apartment in that statement, and yet it was the first statement to be taken. The officer was probably not clued up, neither would the interpreter have been, and Gerry had only had 2 hours sleep, if that.