It's more of a testament to how niche IRC is that a 20K user migration is significant.
I have a hard time believing anything from Telegram because of the owner. But that number does seem feasible.
> People did leave WhatsApp
I don't think people joining Telegram directly leads to people actually leaving WhatsApp. I joined Signal, but am still in WhatsApp because most people are still on there.
* IRC users have the technical acumen to move.
* IRC users aren't strangers to setting things up and configuring them, so the switching cost isn't as big
* There's a shared ethos about IRC, but not so much for an app that reaches everybody on the planet. You can convince one demographic much easier than all of them.
* IRC is interoperable and easy to change servers, unlike apps with proprietary protocols. If Yahoo started doing horrible things with their email, then people who notice would switch to GMail or another host.
For example the SerenityOS project used to advertise on their site that they had a channel on freenode but now they link a Discord instead.
Non-technical users have the same sort of experience e.g. switching cell phone providers (provided your area has number portability), switching between subreddits, etc., and those moves definitely happen.
It's also a testament to the beauty of IRC. "we don't like the change. So here's a viable alternative".
We could say something similar happened with WhatsApp, but the difficulty of moving everyone to another platform is so much harder.