This strikes me as burying the lede -- if TMobile is traffic shaping based on what looks like the speedtest folder, are they also QoS-prioritizing that traffic to get better speedtest results?
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.