The "required" in that sentence should be read strictly: some customers, mainly governmental, wouldn't have bought Intel chips in the first place without access to alternative suppliers (AMD and previously VIA). Intel had to give in.

I read GP to mean that Intel had strong incentive to cooperate in order to make the initial sale. That’s where the customer need was relevant.

Neither company were like they are now back then. Intel needed a second supplier for their chips because nobody trusted manufacturing from a single source provider.

That wasn't the case. Their customers were the military. The second sourcing was required if they wanted DoD contracts.