I've never used Clubhouse, but on iOS and macOS Telegram looks and feels very native.. but better. Snappier, faster, more responsive, yet still looks correct, and works as I expect native apps to work.
Most popular apps don't use native design but rather artistically always a step ahead of native. Think of e.g. the Airbnb, Uber, Slack, Discord, Instagram apps.
If Instagram or Airbnb used native iOS/Android design they'd lose users pretty quickly IMO.
Drag and drop, text inputs, selections, UI elements, keyboard shortcuts, state preservation – none of those worked as they should. I would accept a divergence for a legitimate improvement, but it's just system-wide basic functionality missing.
All of that works correctly in Telegram. There is great value in adhering to the system conventions, design, and using native elements – it's fairly clear what is supposed to be what, a good chunk of the time, and there is minimal context switching as I use other native apps.
Those are services.. where would people go to replace Instagram? Twitter? Airbnb, what other big service has brand mindshare that people know about for fly-by-night unregulated sleeping accomodations?
I don't think iOS/Android design would cause them to lose or gain users.
Uber and Slack look nice but they're rather slow. After a week, the "looks nice" part ceases to matter and all that's left is the annoyance of waiting 200ms-1000ms between every action.