He did get sick, as did Trump, right in the midst of a campaign. That then gave the excuse to the leftist debate commission to cancel the third debate. It might have cost Trump the election.
What cost Trump the election was the deliberate, massive election fraud where the ballot boxes were stuffed in half a dozen swing states to make it appear Biden won with 80 million votes, when in fact many of those votes were fraudulent with nor legitimate voters to back them up and he outright stole the election.
Simple as that.