After reading the article, my understanding is that what was sent in “private chat” was in fact encrypted (for the most part) and can be considered secured (to the degree - something is off and, maybe we didn’t find out yet, how the encryption is compromised).

"Secret Chats", but otherwise spot-on, yes.

I am making a point of clarifying here because Telegram thrives on ambiguity. "Private chat" might mean anything in that system. "Secret Chat" is a specific feature that almost nobody uses but gives Telegram cover to claim they do end-to-end encryption.

But it would wise to treat all other conversations as something that is compromised. Is this a fair summary?

Yes, that's what I would say.

Telegram has access to everything that is not a "Secret Chat". They are responding to data requests. It's unclear what they include in these responses. They are also linked to FSB, through the same Vedeneev guy that owned GNM (the infrastructure provider).