This piqued my curiosity and I did a bit of research. Apparently there are a variety of glasses that can withstand that kind of heat such as aluminosilicate gla

This piqued my curiosity and I did a bit of research. Apparently there are a variety of glasses that can withstand that kind of heat such as aluminosilicate glass and fused quartz / high silica glass. There are also commercial formulations such as Robax, which is apparently some kind of transparent ceramic good up to 825°C (although the temperature / time load capacity listed for that temperature is only 5 hours. Assuming the solar ovens reach a maximum of 700°C you get a far more reasonable 5000 hours). I haven't seen anything about cookware rated to those kinds of temperatures, so I'd assume it's custom-made out of something like the above.