I say sort of because the username isn't quite what it should be, but the post went through.
To make it work, start typing @username@domain, so for example @developer@[email protected] - as you do this, several results will pop up, just select the right one from the list.
It took a really long time for it to go throgh though, so yours might go through yet. Look at the timestamps:
No idea why it's so slow to mastodon, the server's basically idle so it's not a load thing on our part. We federate to other Lemmy instances almost instantly, assuming they're not buckling under load.