The video says that bluetooth operates at higher frequency but about 1000x lower power, which makes a lot of sense. Having a small device near your head broadcast a message a few feet seems much lower risk than having to broadcast all the way to the nearest cell phone tower.

Another thought: my phone and provider (Pixel 2, Google Fi) will automatically use WiFi for calls when available. I wonder if that mode is significantly lower radiation for the same reason (shorter distance to cover, so the signal doesn't need to be as strong).