Prove? No. Let's look at the quote we are discussing again

>"These investigations suggested that large loaves made with water and flour might have been baked on these trays, placed in domed ovens for about two hours at an initial temperature of 420°C."

That's why they are using words like "suggested" and "might". When dealing with ancient civilization often all we have leftover are tools that survived. It's not like we have cookbooks from that era. So all we have is guessing what they might have done, based on attempting to solve the same problem with the same tools.

Even if we did have cookbooks from the era, they didn't have thermometers. We'd have things like Cai Xiang's Record of Tea.

If you see tiny “shrimp eyes” in your water, he wrote, the water is roughly 155°F. If you see “crab eyes”—a slightly bigger bubble—then you’re reaching 175°F. Larger “fish eyes”: 185°F. String of pearls: 195°F to 205°F. And finally, when your hot water has reached a rolling boil, or a “raging torrent,” you know that your water is 212°F."