The following units can be constructed at t3, even without the production structure from their specific factions:
That is, if you have a psi-plugged pandora hub and a foehn war factory, you'll have access to marauders.
Oh that was Prerequisite.Override logic. Not entirely sure if it's intentional or not, but the line did only includes Psychplug (for example) for Marauder etc.
It was probably intended so you can still build Tier 2 if you have PsiCorps tech engi-stolen from another player for example
I don't think it's an intended feature, so long as you can't build Mirage Tanks even with both Shield Command and Tech Center captured.