Since McSweeney is credited for being behind Starmer, this will be a big blow for him personally and professionally. He will also be seen as a more isolated, vulnerable figure to the various circling parties looking for his job.
But Starmer can throw as many people under the bus as he likes. He can try and pretend he's sorry for believing Mandelson's lies. As opposed to accepting responsibility and apologising for that. Even if McSweeney says it was all him, and Starmer nodded it through, only goes so far.
But all the documentation, and trails, should be getting reviewed. The extent of Starmer's involvement will be in there, as well as where others fell on the issue. Whether or not, we'll get to know it, might be an issue though. At least Starmer didn't get to choose what to review/release, as was his first plan.