No one is talking about the impact of kids seeing parents read on phones instead kids seeing parents read on books. When kids see a parent reading a book there's no doubt as to what they're doing. They are reading.

OTOH, even if a parent is reading the most wholesome text on the most wholesome app, the fact that they are doing it on a phone admits some possibility of confusion in the eyes of the child observing. There is a nonzero chance that it could be interpreted by the child as the parent watching a video. Or playing a game.

Children have a very powerful desire to imitate, and it should not be ignored in the debate on literacy.