You have already pointed out the similar question about magical nets, where it quotes from the DMG:
Most magic items, other than potions and scrolls, have resistance to
This would indicate that the Portable Hole has at least resistance to fire, even if made of cloth.
I would say it is a DM decision as to whether an item with resistance to fire also means that something is not flammable (personally I would say not, as a portable hole is quite an extraordinary item so I would say it would need a more extraordinary means of disposal).
Also worth noting that, generally, a creature's equipment is not subject to damage unless specifically noted (to avoid player frustration if nothing else), so the hole would at least need to be unattended for the potential firestarter to try lighting it up.