Hm I thought that's what it was, a shame. IMO 'WebViewHandler's should be distributed with browsers, and then when you choose a default browser you've also chosen a default 'WebViewHandler'.
(Disclaimer: I work for Mozilla but not on this)
And finally (and this applies to a standalone GeckoView package without direct API compatibility as well), end users being able to update the Webview independently from the apps using that Webview package means that you're suddenly operating under much more severe backwards compatibility constraints.