Unless you have a relevant trademark, there isn't really much you can do here
You can rant about it and get that rant read by people who make branding decisions. And sometime in the future someone may say something like "even though there's nothing legal stopping us from re-using this person's existing reasonably well-established online identity, there may be negative consequences, so maybe we should keep looking"
The charge is rudeness, not IP or Trademark violation. A community addresses rudeness by calling it out.
You don't need to formally lodge a trademark application to gain protection. At least, not under UK law. I imagine it's similar in the US.
Not only that, but he can clearly demonstrate both confusion and priority, in the same field. I don't think it's particularly clear-cut which way a judge would go.
It is not as strong as have a federally-registered trademark.
But, as others have pointed out, the problem is in showing the likelihood of confusion.
It's _possible_ that this musician could prove this, but I believe it can be quite expensive, and there's no assurance of winning.
My gut feeling is the the idea of confusion is not so clear cut. If you apply for a trademark you need to specify the specific field of application, and there are quite a few and remarkably segmented. Even having a registered trademark as a musician may not stop another from getting a trademark on a tech conference.
His best bet might have been to try to ride the wave of inadvertent attention, promoting his stuff to people on Twitter who followed him by mistake.
Perhaps some number of them would have become fans/customers.
But I can understand the anger when you think someone is stepping on your identity.
You have to use the trademark "in interstate commerce"[1]. Simply registering a domain or a twitter account is not commerce and you get no protection until its used in commerce.
He doesn't really have any claim to exclusive rights to this particular mix of words.