Yeah, but how long can it stay that bright before dimming all the way because the iPhone gets too hot?
Not going to be long, likely at best 10 minutes under modest usage. If you’re playing a hi def movie or especially a graphics intensive game, it is likely to be just a handful of minutes. But unless it is one of those deformed gaming smartphones with active cooling, no passively cooled phone will sustain high nits out in a direct hot summer sun. That’ll be true of other smartphones too.
Usage rule: personal electronics are not meant for more than relatively brief direct summer sun exposure. Direct summer sun just on a battery, especially one at 100% or only a few percent left, is no way to treat a battery (other parts/circuits unable to actively cool themselves and lower the temperature is no good either).