But that's not the point. The goal of the discovery spec is that the user can enter an email and sign in with an OpenID connect provider of their choice. If I need to do an MX lookup and guess what identity provider they are using it doesn't solve the problem of needing to maintain a list of supported providers.
True, the "well-known" path seems to be in a random location but maybe that's a problem with the spec, you might expect it to be off the root of the email domain.
Facebook: https://www.facebook.com/.well-known/openid-configuration