I think other commenters have already sufficiently explained this, but just to make it clear: Any wire functions as an antenne, also in a normal circuit where it is connected to something. The length of the wire will cause more radiation from some frequencies, but some designs will have wire length that matches some of the frequencies emitted by the regular communications performed (probably most of them actually - you rarely send random noise over your serial communication channels).

Yep - I totally understand that. I wasn't commenting on wires ability to act as antennas in general - I was commenting on the addition of this wire to the UART for the purpose of wireless comms.

That addition is what makes your original point wrong : "Nothing is happening here that doesn't happen already with normal use of the UART"

I doubt the gain of a jumper wire is enough to cause any "damage".

You might well be right on that. "damage" would be good to define in general for these kinds of things. I'm guessing the FCC regs talk about this to some degree.

So, reality is, this is likely very low-power and likely low-impact experiment and likely not cause a lot of issues with nearby devices that are designed to accept all interference and continue to function or fail gracefully.

But IMHO, it is a good idea for anyone doing these kinds of experiments to think a little bit about RF design and the impacts devices might have ..hence my other comment about the value of HAM technicians license & training (which is fun and pretty easy).