RAW immune to psychology does not make you immune to spells unless the spell says something like 'causes fear' or 'immune to fear is not affected' or something along those lines. If the model in question says something like 'auto-passes all ld tests' then they would, of course, be immune.
Possibly it was an oversight and they *intended* to make those immune to fear immune to the spell, but they haven't.
For what it is worth, in my group we play it as RAW and the spell is not overpowered at all. Indeed, it is only ever cast if the caster literally has no other option. YMMV.