This is somewhat disingenuous as the arguably main Google product (gmail) allows enumeration.
It’s otherwise trivial to test for the existence of accounts on email identity providers, so there’s not much point in even including them in the list.
None of the other platforms in that list are (potentially) self-hosted software.
There is no Central Discourse Platform - site administrators are free to choose for themselves.
That said I would agree with giving a nudge to site admins to choose good settings by default.
Maybe a knob admins can turn (I hesitate to suggest adding this to the wizard) to tighten down settings like this easily.