It's really not clear to me as he kind of backtracked in the next message by saying that he was writing under the role in the FTP team, rather than as Project Leader of Debian. My interpretation of that is that it is his opinion, but not a statement of policy. I'd be pretty surprised if they allow SSPL in Debian, but I don't think this is the statement that disallows it.
The FTP team is the body that ordinarily makes binding policy decisions about licenses in Debian. It's not typically the project leader's job, so I read this statement as saying "despite the .signature, I'm speaking in my role as a member of the FTP team, not the project leader overruling the FTP team."
