"Volt Nut" is the term for this. There's a web site/mailing list:
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
And yes, "Time Nut" is also a thing...
I agree; all of my Fluke meters are in-spec by quite some margin, and mostly they all agree plus or minus a couple of least-significant digits. My 2015 THD agrees with them, as does my HP 3468A.
But one can be confident with a large-ish sample size. What if you only have 2 or 3 trustworthy meters? That's when the doubt creeps in, and that's when a reference becomes a useful project. Also, it's useful for buying second-hand at meets, etc (although such a thing would need more functionality to test the AC ranges, plus current and resistance, and wouldn't need such precision). So, I can see both sides
