Yep, but that's the "open source" Android app, where the irregular code dumps are often weeks or months behind the released Play Store version...
And it uses openstreetmap tiles instead of sharing location data with google, and a few more such modifications.
What percentage of Android users, or to be generous, Telegram users, are running Fdroid?
it's not a huge investment, and does not cost any money; market share really doesn't matter here when you can get in easily. you also can download the app on their website without installing their store app.
also keep in mind that telegram definitely can distribute their app on their website, like they do with desktop apps.
If you block Telegram from the Play store, suddenly most of them.