Intersection Observer has been implemented in Chrome since 51, and Opera since 38. It's currently being implemented in Firefox ( https://bugzilla.mozilla.org/show_bug.cgi?id=1243846 ), and is "likely" from Edge (https://developer.microsoft.com/en-us/microsoft-edge/platfor... ).
in-view.js's API is certainly less likely to be the cross-platform API than Intersection Observer. Also, plenty of APIs are implemented by browsers at the editor's draft stage.