“Yes, I know. I know who’s the Thing and who’s not in the very end,” he said during an interview with ComicBook.com. But if you were hoping for any elaboration, then keep on hoping. “Nope,” Carpenter added when probed for more information. “Cannot tell you. Sorry,”
It was pretty much the same answer he gave SYFY WIRE last summer: “I know, but I’m not telling you … I just feel like it’s a secret that must be kept. The gods came down and swore me to secrecy.”
Hard to operate a torch with a slashed thumb would be my guess. I’d chalk it up to a cinematic choice for brevity and interest as well.
Also consider that this blood test was happening at the highest point of paranoia, no way they would let Mac get away with using a different blood source. Didn’t Blair (or someone else) destroy all the blood that was being used for the previous immunology test at this point? Where would Mac have gotten his own blood from if not from himself?
Another thing they don’t show in that scene is a blood bag.
They never show who destroyed the blood which is why this stays semi plausible. It does make sense that they would need to see everyone draw their own blood and you could make a point for brevity. But again, they never show it and even some sleight of hand could have been used. The thing in this movie was very crafty and only showed itself when the time was right. Considering all that happened off screen it’s hard to say.
I know also that John Carpenter saw the 2001 game as a direct sequel to the movie as he never expected there to be a second movie and at the end of that Macready is the helicopter pilot that flies you back to the mainland and it’s assumed that he is the thing. Take that for what you will.