Genuine question: what happens if they refuse to release? How is it enforced? If the answer is going to court, who sues them? EFF?
Some kernel developers have assigned their copyrights directly to the SFC, and some have signed agreements for SFC to do this enforcement on their behalf. (see https://sfconservancy.org/copyleft-compliance/ )
Obviously there are many people and organizations who own copyright to parts of the linux kernel, and any of those can attempt to enforce the GPL on the kernel in court.