Earlier quoted context omitted.
Two thoughts: - How would you provide the functionality that hashtags do on the public timeline? - How would users know which nodes they could trust? I think email and Sms are pretty weird user experiences that have succeeded in part because there was no alternative.
Twitter is already a federation of servers and services. It doesn't seem outrageous to imagine another server or cluster of servers being owned by a second party.
Perhaps I was wrong to say that federation is in principle ruled out, but I do think that it must be invisible to users. - Including namespacing usernames to nodes within the federation as with the @ sign in email.
I believe this is true in spite of the technical beauty of federated solutions, and it's exactly that beauty that's lured so many onto the rocks.