Sorry how could you pirate "the online thing that runs in the browser"?
The problem is that recent OneNote versions (at least on Mac), require Microsoft signin and can't even open local note files.
Well, you can't, that's my point. That's why it's so great for Microsoft: Nobody to pirate it any more, means more revenue.
Office365 still has a desktop executable, parallel to its online program. It's not all browser-only. I think that's why you two are talking past each other.
Doesn't help that I've stopped using Office in favour of Open-/LibreOffice about 7 years ago or such :).
They're barely desktop apps - they're essentially bloated javascript heavy web-frame like apps that perform poorly, have many bugs (IMO 7~ years of use) and require login to Office365 / Azure online.
(While this is almost certainly the case for Office, a well-known leader in its category, it should be noted that this logic is not accurate for all products - Increased opportunity cost to the legal consumer and lack of piracy externalities such as network effects mean "always-online" requirements may be detrimental.)
OneNote for Mac is free.
It was just a SharePoint component. You can easily pirate it.