The problem here is that they didn't log in at all.
- on Stack Overflow, registered user with a certain credential
- on Super User, unregistered user
They posted as an anonymous unregistered user. If they don't even attempt to log in, we can't match their accounts.
Basically they need to click "log in" and use a matching credential for this to work.
Update: if we have any association at all (the user has the same account), we'll correctly match up all their content going forward. In the past if a user was merged after content had been migrated, users on the losing side of the merge were out of luck, since their unique ID changed to the new master and that's what we'd be looking for when seeing which posts they should own upon first logging in.
Now, with everything costanzafied, we'll search not only your brand new shiny account ID, but all previous IDs as well (any that have been merged in).
For the case in the question, it is possible to recover the post if you're still "logged in" as the unregistered user (the owner of the original post) and register. Then, logging into the second site will recover ownership of the post. If you've cleared cookies, etc, and aren't "logged in" as the unregistered user anymore...we (still) can't really do anything to auto-recover ownership there.