I wondered the same! Not with NeXTCubes, but I used to have a Sony VAIO N505X at the end of the 90s. A beautiful, tiny laptop in a magnesium case.[1]
I vividly remembered all these demonstrations of burning magnesium strips in school, and sometimes wondered whether the case of my laptop would react the same. Of course, I liked that thing, so it always remained a question.
[1] Current pictures found on the Internet don't do it justice at all, since the paint and materials seem to have degraded to something very ugly by now.
I still have my 505 Vaio. The rubber feet melted into sticky gel years ago. I put stickers on the lid so not sure what state the paint is in. Battery died a long while ago. Pretty sure the alloy these things are made from is not easily ignitable due to being close to electronics. And to make the product less expensive by alloying with cheaper metals.
I vividly remembered all these demonstrations of burning magnesium strips in school, and sometimes wondered whether the case of my laptop would react the same. Of course, I liked that thing, so it always remained a question.
[1] Current pictures found on the Internet don't do it justice at all, since the paint and materials seem to have degraded to something very ugly by now.