From the IWC IRC channel:

tantek BOT [3:43 PM]
shaners, the h-x-username is just part of the auto-linking - nothing to do with RSVP

[3:43]
though I could see how it might look like it's an attempt at invitations or something

[3:43]
I think barnabywalters asked for it, maybe for styling purposes? maybe I should just use some other non-uf class name
worth re-assessing

Shane Becker [3:44 PM]
oh i see

[3:47]
i think using a non-uf classname would be less ambiguous

tantek BOT [3:55 PM]
yes that's good feedback. can you file a gh issue on that? re: auto_link function