It allows Telegram users to hide in plain-sight, within the noise of other Google Translate web users.
I'm pretty sure that using the official pre-built java SDK, as suggested by the author, would allow Google to cluster the content of Telegram users (since app-specific id/token should be sent).
Other than that, a great read and kudos to the author for shedding light on it.
Edit: typo.
Yes, Telegram fakes the user-agent, but the rest of the request still looks very different from a request an actual browser would do. (No referrer, missing headers, different connection pooling behaviour, possibly different TLS and HTTP2 behaviour, etc).
So if Google is doing any detection for browser vs non-browser requests, those requests should show up as suspicious.
On the contrary - it's the most stupid thing to do. The only result will be their users wondering soon why this function is broken.