I think I know where they're coming from as I used to have a similar wrong model. I thought strength = more muscle cells and endurance = just better heart/lungs to deliver oxygen and clear waste like CO2 and lactic acid.

Turns out muscle fibers mostly grow bigger rather than more numerous, and there are different fiber types (slow-twitch vs fast-twitch) that adapt based on how you train. So for the same muscle, an Ironman runner and a guy doing heavy low-rep squats will develop different fiber characteristics: you can't fully max out both.

I'm simplifying, but learning this changed a lot about how I understand exercise at the biological level.