Telegram isn't as private as iMessage, but that doesn't mean it's not as secure. Security-wise, exploiting iMessage is easier than exploiting telegram, since iMessages has some special privileged access. Security doesn't mean privacy.
Source? It's always been my understanding that while smaller players do this, the major ad players - Meta, Google, etc - never do this: Data on everyone is their greatest competitive advantage, and they don't want to give it away!