That paragraph might be clearer, but Simon is referring to supporting logical replication from 9.1 to recent releases, which essentially means backporting logical decoding (added in 9.4) back to 9.1. There's no way this could get to official 9.1 branch, as that'd be a major risk / behavior change.

Making "zero downtime" upgrades possible is part of the whole logical replication effort - both within the community and in 2ndQuadrant in particular.

Petr Jelinek actually described how to do that using UDR (uni-directional logical replication) in 2014:

https://wiki.postgresql.org/images/a/a8/Udr-pgconf.pdf

There might be a newer talk somewhere, I'm sure he spoke about it on several events.