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That is a great point, but kind of outside the scope of his article. I think using the /speedtest/ folder as a test across the same resource would provide some clues.

s3/asset.jpg s3/speedtest/asset.jpg

Of course, they might whitelist using speedtest but give priority based on originating domain (which would obviously make a billion times more sense).

Maybe. Louis Rossmann demonstrates a definite difference between speedtest and youtube[1]. The comments suggest it might be "binge-on" throttling, not favoring speedtest per say.

[1] https://youtu.be/2ImKF0fly8s

It is a well known fact that many ISP's cache well known speed test sites.

I truly believe that the speed test sites caught on to this and changed from sending a static file to a randomly named file.

Now they probably just prioritize the speed test sites.