I suspect that "president' was left off the list because nobody who had a hand in drafting Section 3 imagined a scenario where the president would join an insurrection against the government he was presiding over.
I don't think it was an intentional omission.
The problen with a list is that it's hard to make it exhaustive. That's why we have a 9th Amendment saying, "The list here is not exhaustive."
I do think the omission gives wiggle room.