I believe you are confusing WebXR with Browser-based AR solutions. WebXR is a native layer iplemented by the browser. It can be mocked, but usually AR solutions like 8th wall (for example) don’t support WebXR or implement its API, but provide their own proprietary API.
WebXR is supported on most (new) Android devices. It is true that some older devices won’t have AR enabled, but I assume that AR solutions that are JS-based won’t run very well on those anyhow.
@ericwood73 is 100% correct in everything 
Apple doesn’t support WebXR on iOS.