If Amazon is confident the last election was fair, they would presumably see the current push as procedural nonsense designed to stop them from winning, and would be concerned that more redos just mean more opportunities for trickery. Would you feel the same way if it were Amazon calling for a do-over after a union victory?
There's an asymmetry of power here though. Amazon, had they lost the vote to the union, asking for a redo would be suspect. But because Amazon has the appearance of all the power and control should welcome redos.
I don't really see the asymmetry. The union campaign was run by a 60,000 member affiliate of the AFL-CIO, supported by multiple Senators, and arguably (he was vague about it) by the President of the United States. We could focus on Amazon's greater power to influence workplace conditions around the election, or the union's greater experience and pull with the NLRB. But really, the bottom line should be, both sides are large and powerful enough to meaningfully worry about the other side playing tricks.