WiFi signals can measure heart rate

This is just a dedicated RF emitter combined with a dedicated receiver. The fact that is it uses WiFi hardware is probably just because that's the cheapest and most available hardware for the researcher to work with. There is no indication in the article that the WiFi can actually be used for transmitting real data at the same time; that a non-dedicated WiFi source can be used; that it works when there are many people between transmitter and receiver.

Therefore the ideas that this might apply to real-world situations and use existing WiFi infrastructure, are a stretch given the information that's been shared.

It basically doesn't seem like a big deal to demonstrate what has been demonstrated.

Research doesn't always seem like a big deal. In this case, using CSI extracted from standard wifi packets (beacons, data frames, etc.) from commodity hardware is the core of the "big deal."

In principle, any packet that carries data can also be used for sensing, though, as you mentioned, this isn't what the researchers demonstrated. However, for years, this kind of thing was studied using special multi-antenna Intel cards to get a clean signal. Getting this level of accuracy from such a low amplitude signal from a single antenna on commodity hardware like an ESP32, is the actual breakthrough. It proves the concept is sound before tackling the much harder problem of using a standard home router amidst other traffic or isolating multiple targets in a room.

Mmwave heartbeat sensors are like $2 at retail pricing. This is commodity stuff, I fail to see how adapting it to a new radio is any kind of big deal.