pop assembly cost denotes a modifier which influences the amount of minerals consumed by the assembler jobs every month, not the amount of accumulated growth needed to spawn a new pop. there is no modifier in the game(vanilla) that i'm aware of that influences that. if you want more/less pops you need to modify the speed at which growth gets accumulated by the way of the mass-produced or custom-made traits.

I had to double check this. I can confirm that you're at least partly correct. I started a vanilla game with the Mechanist civic, and gifted myself robomodding with the console. I created a luxurious robot species, and looked at what happened when my planet started building them. The accumulated growth needed to complete a pop doesn't change, but neither did the upkeep cost of the Roboticist job. As far as I can tell, it doesn't have any effect at all, but at least it's not a bug introduced by this mod.