I think the idea was that China cutting off TSMC would prevent TSMC from accessing ASML fab tools and maintenance, which would quickly degrade TSMC capabilities.
Edited to specify China as the one who would potentially cut off TSMC from the rest of the world, which would also cut off ASML->TSMC.
Edited to specify China as the one who would potentially cut off TSMC from the rest of the world, which would also cut off ASML->TSMC.