To be fair, almost all networks that Pidgin could connect to (Jabber/XMPP being a notable exception) used proprietary protocols, and their operators heavily discouraged third-party interoperability.

The difference is that there wasn't any way to do robust hardware attestation at the time (which is what Apple does to frustrate Beeper-like iMessage interoperability), so the reverse engineers usually won.

Here's an interesting story from that time: https://www.nplusonemag.com/issue-19/essays/chat-wars/