Read Correctly using bookmarks (instead of reposts) by Jamie Tanna (jvt.me)
As I've embraced indie post types, such as reposts, I've noticed that actually I've been using them wrong.

Looking at https://indieweb.org/bookmark#Repost it appears I've been conflating a "retweet" on Twitter with a "repost", thinking they were the same. Alas, they are not, and it makes more sense to be a bookmark.

I've since updated the posts using the wrong type and will get things right next time!