"should dict preemptively make sure it doesn't accept a sequence of sets"

No. The person on StackOverflow made a mistake in their code: they used an unordered data structure (set) instead of an ordered data structure (tuple):

    dict({i,j} for i,j in enumerate(lst))

It is a waste of time to slow down the dict constructor to check for something so unusual as this. Everyone will pay the cost of the checks and virtually no-one will get any benefit.

I'm closing this as "rejected", but if anyone disagrees they can reopen it.

By the way, the best way to initialise a dictionary in this situation is to avoid the unnecessary generator expression and just go straight to enumerate:

    dict(enumerate(lst))