What is a protocol, really? In a literal sense, you can say only something that's IRC compatible is IRC. But in a more figurative sense, aren't Discord communities merely using Discord the way folks use to use IRC? Isn't Discord just centralized IRC, then, because that's how people use it and what they're using it for?

Centralization poses censorship and control risks that IRC doesn't, for sure. But it also adds ubiquity. That's key for a community.