Think the difference is between "using your phone", meaning calling someone and audibly talking to someone, thus disrupting the movie for others, and "checking your phone", which is just looking at it, though inaudibly, though there might be some amount of light pollution depending on the screen brightness. First one has and will continue to be taboo, second one however, in my experience, used to be viewed with distaste, but is pretty common nowadays.

Using your phone used to mean talking. Now it means silently browsing and that's not so bad.

Faux pas or not, it now happens much more than it used to.

I have no idea at all what lost Xanadu theater you found where people don't check their phones. They weren't saying they like it or that it's actually ok. The word "apparently" tells you that this is something they have observed happening and were surprised to observe.