I hope this will be taken as non-partisan, but, at least from the perspective of the UK, Trump was the one saying "the election is rigged". Was that reported in the US?
Yes, and back when Trump was one saying it, it was treated not just as a conspiracy theory but as a dangerous attack on democracy itself. Both social media and the press were insistent that this was ridiculous and impossible.
Yes, only Trump said it and many outside his world strongly rejected it.
He was, when he thought he would lose. It's just as bad for the left to make those claims now as it was for him to make them then. Attacking the legitimacy of elections without evidence is bad, full stop.
The author of the article agrees: He says it's unlikely and wants to check the evidence.