People have been claiming for years that US phone calls are subject to routine computer analysis (Echelon); these days that's a relatively cheap thing to do with LLMs.
I suspect the literal runners solution is what's happening, although that's also very dangerous when the police control the streets.
And don't forget the whatsapp group chat classic: secure communications where at least one person in the group is leaking them.
If you simply list a location and time, it's hard to suss out whether it's a coffee or a protest. And there's a limit on how many people can be surveilled in real time, with the focus probably being on organizers not attendees. You're correct it's a possibility, but as a practical matter they can't listen to everyone, all the time -- but the key is to organize some "event" that overwhelms the regime, or days or weeks later they will possibly get around to your intercept and give you grief, yes.
But if the internet has been cut off and the bodies are piling up, sometimes you might choose to take a calculated risk.