This definitely works in Marvin, as long as you and the person you're sharing your annotations with are both using Marvin and the exact same EPUB file containing the book, as mentioned in the other thread. Marvin, as of today, offers no server capabilities for syncing or sharing annotations, but this manual way of emailing the annotations file to someone else has worked fine for me in the past.
I'm not even sure Hyphen or MapleRead offer the option to share one's annotations with someone else using an annotations interchange file. They do allow the (buggy) export of annotations via email, but I'm not sure if there are any other, more advanced annotations export/sharing options apart from that, in Hyphen or MapleRead. But, as opposed to Marvin, those two do allow the server-based syncing of annotations between various devices owned by the same user.
As can be seen, each app offers some things that the other app(s) do not, and vice versa. It's perhaps natural to a degree, but it's also frustrating.
Because, of course, the ideal situation would be where you have one e-reader app that offers
all the useful features, not just some of them.