That's what I mean, all skepticism would fly out the window if a materialist soldier had his mind invaded by a psionic one.They do. I've had psionic materialists before.
The only real problem seems to be that just seeing proof of psionics isn't enough to make materialists go down that line of research. They need an expert scientist (or, I think the 2.2 patch notes said a psionic scientist works too).
Having any psionic pops, or seeing a psionic entity, or fighting against psionic troops etc, should be enough.