Different activities have different contexts for what "being able to do it" means.

If I said I can't drive, that means I have no license and probably think there's too high of a chance of something going wrong if I drove. That doesn't mean that idk how to steer, hit the gas, or brake.

If I said I can't walk that means I'm paralyzed.

Swimming is somewhere in between those two on the spectrum of "what does it mean when I say I can't".

I'd say that I am currently a bad/weak swimmer because I had some independent practice recently, but there was definitely a time where I'd say I didn't know how to swim even though I had taken lessons at that point.