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.)

[ignore, I've misread] ~~In that case, you must have iCloud+ subscription~~

I tried this and RSVP'd with an email that didn't have an Apple account, and it asked me to created an Apple account.

I originally created the event using my own Apple account which definitely has iCloud+. So how do I create an event that someone without an Apple account can RSVP to?

I don't know - I was able share a link and RSVP without an account. AFAIK I enabled every option other than the shared album.

It does prefer contacting via email, so it did an email verification via mailed PIN, and then attached that email to the guest list from the link.