Alas, a humble pro bono user. SF writing is sadly not as lucrative as I wish it could be. 
I tried embed any origin, but alas, no change in behavior.
(I’ve also looked at the source code for the WP Discourse plugin, which seems to only be talking to the Discourse API to do its thing and not using the normal embedding support at all, which now I’ve seen how Discourse embeds look in Ghost is not surprising - the comments in the old WordPress looked and behaved differently.
So now I have to wonder if there is any embed URL recorded for those topics on the Discourse side at all - although that begs the question of why I’m not simply seeing new (embed) topics appear now instead of this breakage - and if there’s any way I could add one to them?)