That does not mean my complaint is invalid.
I also used to when staying at self catering cottages tell the owners if there were issues that could become problems e.g. the radiators are a bit noisy, it sounds like you have air in them, or even - this is not a complaint, just to let you know your shower is a bit temperamental. I have stopped saying this, even though I am actually trying to help them.
But if someone has a complaint, the fact no one has ever complained does not mean I am wrong to complain. So yes the cottage was dirty. The fact no one has ever complained before might be because this is the first time it was not properly cleaned.
Never respond to a complaint on this way.