I think what @deadsuperhero@lemmy.ml is trying to explain is that Mastodon and Lemmy for example may be able to implement Sup as a part of their softwares too. Currently if you try to DM someone on Lemmy, the options are a Matrix redirect, or Private Messages designated as not secured.
Though I do wonder about the viability of this. People would still need to set up with Sup to receive messages, at least as I understand it. As a result, this would run into a similar problem as matrix, just without needing to use a different federated server and login I suppose.
This would also require that project leads for AP softwares would want to accept the service as a part of their distribution, assuming it even is implmented in such a manner.
TL;DR it's hard to say anything for certain when details seem to be slim.