Active FAANG employee here, so a bit of my take on it. Personal opinion, of course, and not any sort of official Amazon statement.
Management DOES do a lot to prioritize/delegate/lubricate channels of communication, etc. That is their primary role.
But you have more engineers than you have managers. You have more people providing a more varied set of insights into what should be prioritized, what general work needs to be done, what opportunities exist, etc.
To a certain extent, you have to be willing to let some people sometimes be working a little sub-optimally to allow the autonomy that results in making some of the crazy cool products and services these companies create. That autonomy being available is one of the big things that drew me to Amazon, and it's something I believe I've taken full advantage of to the company's benefit.
Giving people the latitude to run projects and create PoCs before getting full buy in from management allows people to be creative. But yes, if you've spent a year working on a project, you should be expected to show the results of what you've done. Some stuff is more ambitious, sure - but even when the results aren't immediately obvious, you do need to be able to explain the potential and what data backs up that potential and the ability for the project to reach it.
From my experiences over the past five years, being able to find opportunities for improvement is hugely important to the culture and the promotion process. You need to be able to identify these opportunities, and that generally means some sort of metric is available to work from, and measure against as you try to improve it. (Of course metrics and statistics can be gamed, but I'd argue it's not good to go through life assuming everyone is a bad actor). It's a skill that sometimes needs to be developed, and it's something I work with people on to help them with the process, but I think it's a very good thing in general. I've seen a lot of positive come out of it.
My two cents, anyway.