They're not starting from scratch. IBM demonstrated their 2nm process over a year and a half ago, they were the first to do so. They're at the stage where they're getting it from the lab to production.

IBM's not a newcomer to the fab business, they've just been mostly focused on producing their own chips for their own enterprise/datacenter equipment rather than mass market equipment. I'm betting that this partnership is IBM eyeing the fab-only business model; or looking to get themselves fabless like how AMD divested themselves of their fabs and spun off Global Foundries.