It's got to be Daryl: "They've asked us not to name this anonymous singer. Fair enough. But maybe you, the very knowledgeable listener, can take a guess. ...Maybe someone who fronts another post-hardcore band you may have enjoyed."
I spoke with Geoff at a solo show a few years back. I brought up UN and he mentioned Daryl contributed a few guitar parts but did no vocals and didn't have as large a role as everyone initially thought. He also said he wanted to tour with Deafheaven when The Next Four Years dropped but it fell through haha.