As long as they’re not using Russian-purchased sims to manage and post to the channels, how does this change their security model going forward?

If IStories' reporting on GNM's connection to FSB and GNM's access to Telegram's traffic is correct – and I have no reason to believe otherwise, this has gone through two rounds of fact-checking and these are people who had been sued for "defamation" in the most journalist-hostile, oligarch-friendly jurisdiction in the world (UK) and have repeatedly won – then this means the threat model now includes the FSB potentially being able to:

• figure out where a user is in the world just by observing their Telegram network traffic, live or close to live;
• with some additional analysis, based on timing and packet sizes correlation, probably figure out who that user is communicating via Telegram.

Both of these globally, regardless of what SIM card was used to register any of accounts involved, and without having to ask Telegram for any data.

I don't know if FSB is actually using this capability, and to what extent, and against whom. But based on IStories' reporting and on my own packet captures analysis it is entirely possible for them to do so if they choose to.