Probably because he wore a wetsuit :)

I think all they (somewhat clumsily) allude to is that without having to work against gravity, the heart’s load is much reduced and so it reduces in size.

The clumsy part, at least in the reporting , is “why” swimming would be anything like being “weightless,” since we don’t cease to weigh the same just because we’re in water (even in orbit we’re under similar gravitational forces but a spacecraft and everything in it is “falling” at the perfect rate). The answer has nothing to do with buoyancy or swimming per se, but merely that a swimmer is prone, and so the work and pressures required are reduced compared to pumping blood from feet to head while standing.

From this perspective, swimming is no worse than laying in bed, and being active horizontally is better than sleeping.

I helped out in a study that substituted space for the subject sitting in a tank of water. I don't remember the results but he was looking for differences in neural drive when not influenced by gravity.