That doesn't make sense to me, siesta wastes precious daylight. If anything, the availability of light at night should have promoted daytime sleep, if there was any influence.

Growing up somewhere with a Mediterranean climate that has hot summers, I’ve always assumed it was to sleep through the heat before a/c was invented.

There is almost certainly something to this. Sun exposure is dangerous.

How can people sleep during the hottest part of the day? I've never been able to manage that.

You can doze off. And falling asleep is a skill you can train.

(I can nap in hotter and brighter conditions during the day than I can tolerate at night.)

Lights help extend the evening. Such that you sort of barrel through a time that would make you tired by just cranking up the light.