I should clarify that the artfifact shown (the crossover blib) is not a glitch as described in the linked article.
The glitches (which the used DAC chip indeed has) have already been removed.
The 0 crossing thing you see is (most likely) caused by DAC chip '1' taking over the positive half of the sign wave to the negative half of DAC chip '2'.
I suspect (no evidence here, just a hunch, I guess mr. Moffat or Jason can correct me) that the chips are in signed magnitude config (hence the 21 bit resolution with 2x 20 bit chips) and perhaps the 'blib' is caused by an extremely small DC offset between the '0Voutputs' of both signal halves where they are 'combined' to create the complete signal.
The 'size' of the blib may actually differ per Iggy as well.
In all cases ... the blib may be quite measurable, because the noise floor of the electronic itself is very low, but is so low in amplitude it will be inaudible.
The practical noise floor, with an actual audio signal on it, will be higher than better measuring/performing DAC's for sure but as mentioned before will still be much lower than any real world recording out there so will be 'masked'.
Indeed as atomicbob already answerred the 'fundamental frequency' of the blib is estimated by looking at the length of a known period (the actual sinewave of 1kHz) in millimeters on the screen and the periodlength of blib and simply dividing those numbers and multiply that result x 1kHz (the used sinewave).
As that 'blib' has 'sharp edges' it will also have harmonics but those are far above the audible range.
The amplitude of the blib is estimated in the same way by measuring (in peak mm's on the screen) a known amplitude (-90dB) and the amplitude of the blib and crunching some numbers.
There is no higher math, assumptions, or theories involved other than quite basic stuff.