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.
Exclusivity is a basic part of business model. Look at PS4 with exclusive titles. Hell, look at your local store with exclusive products only available in their stores.
I would have agreed with you if Apple had done this for a basic feature like calling. But this sure is a privileged feature and there is nothing wrong in making it exclusive to iPhone (but they haven't you see).