The body both absorbs RF, meaning there has to be a safe absorption rate (SAR) and creates impedance with it. It also creates radio shadows. What’s more, larger individuals have more of this effect. At Fitbit there was a guy who I’ll refer to only by his first name — Tim — who was our first port of call for whether or not our prototypes were getting the job done, RF-wise. He was a very large human. (proportionally speaking — he was also very fit!)

Radio waves are not ionizing radiation, so they aren't directly cancerous in the same way as say, x-rays or UV light (UV light is about the threshold for this), but they are still absorbed by water in tissue, which heats it up slightly. There is a limit to how much this is safe, and so radio safety standards impose limits on the average power you can use to transmit in most consumer devices. The standards are stricter for something intended to be carried in a pocket or worn on the body.

I was under the impression radio waves couldn’t interact with biological matter in any meaningful way.

You don't have a microwave oven?

Matter of any kind interacts with radio wave on some level. You might be thinking of how radio waves in the bands and power used for cellular don't have a negative health impact. But they still 100% interact and the human body, being both dense and conductive, absorbs radio waves beautifully.