He could have had a flamethrower.
Or a pistol that shoots flamethrowers that shoot more flamethrowers that are permanently on and have infinite amounts of fuel!
Hey guys, stop scaring The Thing! I'm letting it stay with me until it can arrange transport off-planet and it still has a fire phobia.
Tell it that the fault only lies with itself. If it had been intelligent and invaded one of the many bases in the arctic without flamethrowers, as opposed to the two that DO, it would have taken us all over by now!
It was just trying to prove itself better than the carrot Thing from 1951. I think it succeeded. Besides, it's just going for a while, to assimilate something fireproof, and come back and assimilate everyone.
Except for me; it owes me now.