No it’s not a dark pattern. They’re being as transparent as possible. If you long press the message and click “info” they even explain what each tick means and when each event took place. It’s literally not possible to be more transparent than that.

And before the privacy brigade who’ve not used the app show up, this is configurable. You can opt out of sending and receiving read receipts. And since it’s a closed app with no other implementation, you can’t circumvent that either.