The problem is not that the computer is going to act like a stove and start the cardboard on fire via heat transfer, but that if the computer happens to catch on fire, it's going to spread easily via the cardboard.
Take it from someone who's seen a lot of bitcoin mining accidents in his day.
Ehh, No big deal. There are about 50 different flame proofing treatments for cardboard on the market. You just paint / spray the products on thick enough to penetrate the cardboard, let it dry, and the cardboard will no longer support a flame. I think it's totally over kill, in reality, but it's cheap and easy to do.