After some testing, I figured out it has to go on each individual alien rank.
alienDeployments:
- type: STR_MEDIUM_SCOUT
- alienRank: 5
cheatTurn: 1
- alienRank: 4
cheatTurn: 1
It also doesn't work when the cheat turn is set to 0.
Setting intelligence to 0 does work. It seems to allow them to use psionics on soldiers they saw during their turn or in your turn before it. You can theoretically make your psiweak soldiers hide from either the aliens or your squad, or you can kill the psiweak soldiers, and that will make the psi attacks stop. It's almost what I want, but not quite. It's still a bit cheaty on their part.