This is true, although the public hotspot speeds are usually fairly limited so it shouldn't impact your private network too much.
I use my own equipment for a whole host of reasons, but I also appreciate having WiFi I can connect to more often than not without having to drain my phone's battery using it as a hotspot.
Extra WiFi traffic, even if not coming from your AP or devices, will still interfere with your own AP and devices for the reasons walrus01 pointed out, that WiFi is half duplex and needs to share the same RF space as every other WiFi device nearby.
and even worse in the scenario where the associated wifi clients are kind of far away and at a weak signal (like -77), so they're using more air time for more rudimentary modulation vs. the devices in your house that are using at least 64QAM. there's only a finite amount of airtime in a half duplex system...
I use my own equipment for a whole host of reasons, but I also appreciate having WiFi I can connect to more often than not without having to drain my phone's battery using it as a hotspot.