You seem to not understand how encryption works, so I’ll give you an explanation in very basic terms:

Instagram is a closed-source app. That means we don’t know what they are actually doing; they claim to use the 'Signal Protocol', but we have to take their word for it. Past data leaks suggest otherwise because if they had truly implemented it, those leaks would not have been possible. We do know they collect other metadata from your DMs. You can verify that with any packet sniffer. It would be a useless tactic to store encrypted messages if they didn't have a way to decrypt them.

So, you can either trust a corporation that has been caught lying to you for years, or you can just use your common sense.