Apple will use it's dominant position to create lock in like how they did with iMessage instead of cooperating with other platforms on a common standard.
Oder friends and family are surprised when they want to video call over Facetime and find it hard to believe other people's phones don't have Apple apps.
The app allows iPhone users to create an event. Anybody on any device or browser can RSVP. The event can be shared as a link. Making an event invite app that only works for users on one platform would be pointless.
Also - non-Apple users have been able to join FaceTime calls via. A link for several years.
This product, much like iMessage and others, provides an inferior experience to non-Apple users. It aims to make other devices and operating systems look less capable and cheap.
iMessage also partially works with other phones. This doesn't change the fact that its intention is to create a lock-in effect, as evidenced by internal Apple emails.
Typical HN downvoting because of “muh vendor lockin” without giving an answer as to why exactly this matters for the general population.
It is a fantastic business model.