FWIW there are many transmitters similar to what they built that enable playing MP3 players, phones and other things on old radios [1]. I do not remember what the power limit is but no permit is required. They can operate throughout the entire FM broadcast spectrum. If they are broadcasting good music in an elderly community I doubt anyone will complain.

[1] - https://www.amazon.com/dp/B08L3WX26S/

Several homes in my city put up holiday lights that are synced to music and transmitted over FM. The signal only works if you’re parked near the house.