Just wanted to clarify, WordPress core functionality doesn’t convert a reply webmention to a standard comment, that is done with the webmention plugin. I believe part of that is so you can do a webmention reply to a reply, and as mentioned, the fact that
custom comment type isn’t fully fleshed out. There was work on it, a feature plugin for it, but it has since languished, despite David Shanske’s attempts to move the needle forward.
I don’t think we will see much movement on it until webmentions reach a crtiical mass, hopefully https://micro.blog will help with that.