Yes it very well could be his judgement is compromised (it sounds like it is, with your description "not as sharp as they used to be").
I know its awful your mum must have been cold on the loo and her circulation and fluid levels very compromised after all that time, plus lack of meds, but you couldnt have known, so she suffered and had to be admitted, but this can also lead social care to take the necessary safeguarding action. Whereas, if you were managing it all, getting her up and off the toilet, they wont do anything. I hope you understand im not saying that to be callous, but it is what Ive seen happen.
Given this new event and safety concern in terms of both your mums physical weakness and your dads compromised judgement, a care needs assessment should be done, if it hasnt already recently, but with someone present who can ensure the facts and truth are relayed and not minimised.
A family member frequently fell and forgot to use the bracelet alert or delayed pressing it for hours.........I made sure her frailty and falls team knew this. They referred to adult social care safeguarding then they arranged a bracelet that automatically detects falls. -obviously that wont work for a stuck on the toilet situation. Sounds like your mum might need that kind of bracelet.
Its helpful to keep note of these safety events and dates because it is helpful to refer back to show a pattern where more support is needed to keep them safe at home and to help ensure they are not blocking the care they need due to their own lack of insight. The family member i know would deny falling or argue, for example. I could whip out the notebook in the presence of professionals and say it happened on x date. This is what we found........
Some posters have said about your parents being allowed to make their own decisions, well i would say a decision that only affects them is different and it depends very much if they understand consequences. Did your mum say to dad, dont call ambulance i will be ok.......if she did, she cannot have understood she could end up in hospital. Dad whether he was influenced by mum or not, clearly didnt understand how serious this was as he chose an unsafe course of action, delaying telling anyone. So in both these examples, that would show a lack of capacity.
Cancelling a bracelet alert when your mum has frequent falls in a misguided effort to save money is another unwise decision and that impacts your mums health, it impacts you, and it most emphatically underlines that your dad (and also mum?) have a lack of appropriate insight into risk (thinking they dont need alerts like this, its a waste of money).
I hope this helps, this is the kind of language you can use to argue their care needs with adult social care whether they are self funding or not.
My sympathies, its very hard xx