The idea has been conceived and executed by creative agency Mother, in partnership with Kinggath Creations - a team of creatives, fans and mod developers who make fun and original content that blends seamlessly into the game world.

https://creative.salon/articles/work/ikea-mother-kallax-stor...

It is likely they had the idea they (the people that actually made the mod) could talk a brand into this and IKEA was one of the many brands pitched. But someone at IKEA has to have seen this and say okay this makes sense for it work obviously. Eitherway i would say it doesn't really matter as long as they do fun stuff like this