Impossible to Reassign Posts to a Guest Author

From the ReadMe:

'What happens to posts and pages when I delete a user assigned to a post or page as a coauthor?

When a user is deleted from WordPress, they will be removed from all posts for which they are co-authors. If you chose to reassign their posts to another user, that user will be set as the coauthor instead.'

My Guest Authors aren't showing up in the dropdown box as a valid target for reassignment. Is that a bug, or by design?

When someone leaves our organization, we'd love to reassign their posts to a Guest Author (instead of their actual WordPress user account which gets removed).

Start with the reassignment behavior described in the issue and compare the ReadMe's documented flow with the user dropdown shown when a WordPress user is deleted. Reproduce the case with a Guest Author, then trace how eligible reassignment targets are selected; done means a Guest Author can be selected and the affected posts retain that Guest Author.

Written by the indexing model from the issue text.