Your forgetting the important metric - people who signed up to Amazon Prime and then watched the series. Remember Amazon isn't really a streaming service. It's an online shop and Amazon Prime is really its delivery subscription service. If they can show correlation between a series releasing, people subscribing and watching the series and those people then going on to spend money in the shop then the show is a success.
Also lying to the public is lying to investors with a publicly traded company. Existing investors and analysts will decide on the value of Amazon based on the things that are said to the public. Members of the public may choose to invest in Amazon based on the things that are said to the public. Executives can't come out and outright lie about those details.