I still don't see the problem. Of course the seat got fixed. It is in the manifest. If the OP just wanted the seat to be fixed there was no need to write in at all. People who write in generally seek compensation.

So, the response would have been better if they had replied saying that they regretted that seat 64A was broken and that they have since fixed the seat, thanking the OP for bringing it to BA's attention and have a good day!

The OP clearly stated he did not want compensation.