There is one reason that makes it potentially legitimate to compare screens at different brightnesses when gauging battery life, but not when comparing SoCs. There are screen technologies that adjust color representation and brightness together, to make it possible for a dimmer screen to look brighter (or show up better in brighter light) by sacrificing some color range in a mostly imperceptible range. If you're showing off a technology like that to improve battery life, it's completely reasonable to compare screens at different absolute brightnesses in nits. But if you're comparing generations of SoCs, you should be doing so with exactly the same screen brightness.