I could also be answering a phone call. I think browsing your phone is mostly considered rude by people, but answering a call would be more grey area. Depending on the nature of the meet up and the perceived urgency of the call (which would be hard to estimate for the rest of the people present).
I read parent's comments more as a counter argument to the suggestion more people would be vegetarian if they had to kill their own animals for food, not a moral justification based on "people used to do it".
Striving to focus on important problems instead of just urgent problems would work fine with this method.