I don't understand the point about vertical integration. Amazon use their own hardware and services internally and AWS makes a ton of money of course.
But if you were accounting for the profit or loss of Alexa specifically, of course you would account for all the AWS services used at the price they are sold for externally.
Amazon is not a philanthropy. The company needs to make money and as much of it to satisfy shareholders (who, BTW, are never satisfied).
Having spent tens of billions on something that doesn't add up to the bottom line doesn't go well with shareholders, especially when there's no prospect that it ever will.
But if you were accounting for the profit or loss of Alexa specifically, of course you would account for all the AWS services used at the price they are sold for externally.