First, Amazon's valuation is theoretical. Their valuation is so detached from their fundamentals and so dependent on a narrative about their future performance that Matthew Yglesias describes them as "a charity benefiting American consumers funded by Wall Street".

Second, there's no valid comparison between the A-round valuation of a startup and Amazon, a giant publicly traded company with a decades-long track record and huge cash flows. Mutual and pension fund managers invest in Amazon. To a first approximation, none of those managers directly value startups.