It's true but phones are much more obvious that you have them out and probably filming where the glasses aren't so can be more controlled and at least reacted to. The glasses create more uncertainty about if the person is filming.
It's more obvious, but since people use their phones all the time anyway, you really have to look at the angle they hold the phone to be sure. So it's not super hard to be sneaky about it even with a phone.
Most people are pretty obvious about it if you're looking. It's still the fact they could be doing it right in front of you without you knowing that really makes people hate the subtle-camera-on-face-products.