> Even if they store the data encrypted on their servers and hold the keys - it is not different from plaintext.

That's the important point. Encryption at rest is little more than a marketing gimmick if the same entity also has the key.

Edit: also, Telegram is hoarding this data and nothing prevents them from using it for financial gain in the future. Or selling it/themselves to someone who does.

Better be safe and do not give this data to the intermediaries. Signal does the right thing here.