This was clearly an awful situation for everyone involved. Sorry @randahl and @HarriettMB! Neither of you deserved this.
I investigated and confirmed that this was due to a bug in the bridge. It was always designed to avoid link previews for @-mentions, and usually did, but in a few rare cases like this one, it didn’t. That bug was fixed back in March, just days after the post in question here. Tough timing. Details: https://github.com/snarfed/bridgy-fed/issues/1987
Apologies again for the bug and misunderstanding here.