the odds are extremely low, but the owners may have allergies to certain cleaning products.
Or to cats, if the 'cat flap' was actually some kind of air vent and not a cat flap at all.
Either way, I don't think it's going to just be a case of a half-hour wipe around if the cat was in there without toilet facilities for such a long time. Maybe scratch marks too. It could have run up a significant bill to make good the damage for the home owners. If they've paid for a brand new carpet, I think they'd be horrified to be expected to just put up with it having been used as a toilet by a stranger's cat before they could even properly enjoy it themselves.
And I completely disagree with the poster who said they should bear the costs for damage and cleaning themselves, as 'punishment' for having a cat flap on their own property. Just as absurd as blaming people for having a letterbox and thus exonerating anybody who puts fireworks, dog poo or any other unpleasant/offensive materials through it.