The first case would be an issue yes, although this app doesn't exactly strike me as something that an insane amount of people would be connecting to from corporate networks

It can work okay for apps that don't go to massive scale and where users aren't concentrated in the same place. The chance of another simultaneous user on your block/building/CGNAT isn't extremely likely.

CGNAT is used in places other than corporate networks.

I'm aware of that - I spoke to the corporate network part specifically as the post I was replying to specifically listed that in addition to CGNAT