"The doomsday scenario of choosing between (a) and (b) becomes less likely if we make it clear how bad it would be. We need to provide appropriate backpressure to encourage upstream decisions that support the continued freedom of our users."
In other words, Jackson is willing to drop GNOME from Debian to pressure GNOME developers into not making dependencies on systemd.
But his "doomsday scenario," where Debian has to drop support for all other inits or drop all support for GNOME, is a false dilemma. Debian can still support other init systems, for people who want them, while still letting GNOME depend on systemd if that's what's best for GNOME. Users who don't want systemd can run without a DE, or they can run a DE that doesn't require systemd. (GNOME isn't even the default DE for Jessie, that's Xfce.)
So yes, he wants to make it clear how bad it would be, but he's also the one who's pushing for making it bad. It's not like he's just warning us that someone else will make Debian choose between those two things, he's trying to make Debian choose between them.
"The doomsday scenario of choosing between (a) and (b) becomes less likely if we make it clear how bad it would be. We need to provide appropriate backpressure to encourage upstream decisions that support the continued freedom of our users."
In other words, Jackson is willing to drop GNOME from Debian to pressure GNOME developers into not making dependencies on systemd.