Supposed to give invasion defense of +90
but doesn't add to the defense value of the planet?
What gives?
Or is this adding to the population instead?
Or to some number we don't see?

It's not shown, but invasion defense is used when calculating chance to invade to lower the attacker's chances. same thing with ship modules like the psionic amplifier and prototype mech grant invasion attack.

invasion defense is different from regular defense. It only applies towards invasions but for the level at which most people will get it its very space efficient.