I see this a fair bit on YouTube videos. Uneducated people say “Look at the motherboard”, as they point at any old PCB. A bloke just pointed at the PCB of a broken Xbox controller “ooooh, look at the motherboard” ¯\_(ツ)_/¯
I blame the likes of “iFixit” etc who use incorrect terminology all the time, and homogenise specific parts into the wrong category.
The term “motherboard” implies it had a daughter board, as many computers had. If it’s a PCB that can’t even HAVE that extensibility, the term is nonsense.