As per the demo, in order for Siri to rotate your passwords "for you", you have to open the Password app, go to their dashboard on weak or exposed passwords, and click a button asking it to rotate your password account by account.

So yes. It's off by default. You have to affirmatively use the feature. (This is purely based on what I remember from the demo, mind you. I have not used the feature.)

This one is getting a lot of undue flak. Not only does it require explicit confirmation, it’s also contained entirely within the passwords app which already has access to all your passwords because you chose to trust it.

If you use this app, open it and look at how many entries fall under the “security” section. Everyday another password is compromised and added to the list, just too many to keep up. So, albeit apprehensively, I for one appreciate this feature.