I think your perspective is a little skewed.
Firstly, extracurricular or specific topics things are added to the curriculum because parents fail in their duties as parents. Parents who don't teach children about puberty or hygiene. I mean some schools do toothbrushing lessons and sessions. Does that mean you shouldn't teach your children to brush their teeth?
Second to that, why do you think that there are so many swimming classes etc if schools 1. Did swimming lessons or 2. To an acceptable level? I have four children, it is expensive when you have a lot. So I do understand. There are summer programmes and monthly schemes and lots of options, mine have all had lessons and we teach them ourselves too, two at excellent swimmers, one has severe additional needs and has struggled, he can sort of swim ish but not to the standard of the others. My youngest is 7 and still can't swim. She has had a lot of hours of swim lessons, way above what they are offered at school.
Lastly, school lessons are more part of PE than actual swimming lessons, it's really basic lessons with a high level of pupils for one teacher, its not ideal at all. The school are not responsible to teach your child from a non swimmer to a confident swimmer. It's likely they would only have hot 6 weeks of "lessons" max.
Also, what the curriculum covers is the ideal. Ideally children leave school with those skills, also ideally they have some of those skills before they start those lessons. Like ideally children have a certain attainment in various skills, knowledge etc when they leave but of course that's not the case. My eldest, who also has additional needs, was above average in maths and below in English, is that the schools fault? Absolutely not and the attainment from what he was at when he joined the school (in year 3) and when he left was remarkable.
If Covid hadn't happened your child would likely have lessons but that doesn't mean he would be a confident swimmer.
Yes lessons take money, time etc, it's a struggle balancing it all, have you considered a half term programme? It's a good way to learn quickly.