No disagreement on what you write, but the CMB spectrum is about as useless as radiocarbon dating for deciding which of two Hydrogen maser clocks on opposite si

No disagreement on what you write, but the CMB spectrum is about as useless as radiocarbon dating for deciding which of two Hydrogen maser clocks on opposite sides of a university laboratory is wrong when they differ by ten nanoseconds. However, we can generalize your point and say that real configurations of matter let us make those decisions reasonably and with good accuracy. (The BIPM UTC Circular-T contributors seem to manage fine, as do the various GNSS operators). If the video had taken care to set their experiment in vacuum flat spacetime (perturbed only by the experiment), as the shown Einstein 1905 work <https://www.fourmilab.ch/etexts/einstein/specrel/ https://www.fourmilab.ch/etexts/einstein/specrel/> (top of p. 3) did, it would be better. That's because that setting offers no convenient-to-agree way to slice spacetime into space and time. The CMB does, but we suffer from practical limits on measuring the CMB's blackbody temperature.