And even if it was true, I would not credit the effect on the act of being manually involved but on that egg being from a trusted source. "I know there is an actual in the meal because I saw it with my own eyes". Otherwise, the entire meal would be composed of essentially industrial mystery goo.
Even if this was true for cake mix, it seems a bit of a leap to apply it to IKEA furniture. The simplest explanation that flat packing allowed them to reduce costs seems sufficient. I don't think anyone considers assembling IKEA furniture close to making your own furniture in the way that baking a cake from mix is close to baking your own cake.