[TeMPOraL]

Anyone can RSVP, but only Apple users can fully partake in it.

Also, per sgt's comment below, it seems it works the same way as sharing documents via OneDrive. "Share with anyone, doesn't require sign-in". That is the actual text from the Share dialog in Windows 11. "Doesn't require sign-in". Well, except if you're sharing more than one document under a link - then it forces recipients to sign in with an account. It's even documented in the on-line help for the feature, just not mentioned in the UI. Also, when you share a single document, while sign-in truly isn't required, the link still leads to a login page that urges signing in or creating an account, and just has this tiny, barely noticeable link to access without login, tucked in the corner somewhere.

(I miss Dropbox's "Public" folder from a decade ago. That was the first and last time sharing documents from web drives made sense.)