Yes, but that's the basis on which the site agreed to pay out:

This market will be settled according to the candidate that has the most projected Electoral College votes won at the 2020 presidential election.

He is projected because it is mathematically impossible for Trump to overtake him with any remaining ballots left to be counted.

The actual election will be held on December 14.

(Source: the U.S. Constitution.)

That's correct. Instances of voter fraud must be investigated and resolved, then the winner will be officially be decided.

Exactly, previous elections were fairly predictable in what states each candidate would win. In the past, elections had been predicted incorrectly. Some may not know this, but the Chicago Tribune (in)famously preprinted copies stating that Dewey had defeated Truman in part due to the state-of-the-art election predictions which were straw poles. Ironic that Gallop rose to prominence from this situation and now history repeats itself in a way (meaning that if the poles were correct this election would have been called on election day).

[1] https://www.washingtonpost.com/history/2020/11/05/george-gal...