They're saying if the stock goes up and you get margin-called on the short, you have to sell index shares, you can't just annihilate the Tesla shares with the anti-Tesla shares and walk away.

That depends if you trade cash or synthetic.

I think most people trade synthetic, just because it's faster and you don't have to wait for settlements, but maybe that is different if you trade onshore (I am a foreign investor).

Anyway if you are synthetic your margin is most likely shared between shorts and long on the same instrument, so no, you wouldn't be called.