Official internal chat will be either Slack or MS Teams. Using any unauthorised app to discuss matters relating to the business would be a contract violation.
Realistically, game devs do this all the time in private chats with colleagues that happen outside work, both online and in person. But of course R* either never knows about those or chooses to overlook it.
In this case, I suspect the fact they were unionising was the reason they actually took action on it. It’s not about consistency, it’s about having an excuse to fire these people.
We already know the affected workers were using Discord to organise. My guess is they were discussing work issues there, and Rockstar jumped on that as an excuse to fire them (as it would constitute a breach of NDA).