The reason why his Psak would be different today is because when you walk past a house an you hear the air conditioner go on, you don’t even assume that someone just turned it on. The factors that go into making something a Zilzul Shabbos have changed.
You seem to be conflating maris ayin with zilzul Shabbos.
It’s possible that assumptions can be related to zilzul Shabbos, but Rav Moshe gives a different explanation for why timers are zilzul Shabbos (he says having melocho done automatically on Shabbos is a zilzul, and then explains why leaving food on a blech to cook is different), and nothing changed.