If they were over stating their performance you would have a point, but in the examples above what they are stating is already behind the good semi pro
Not understanding the spec you are referencing to is not a good basis for assumptions. Here is one for the Sound Devices 888 analog input:
THD + Noise: 0.005% max (mic in, 1 kHz, 22 Hz–22 kHz BW, trim at 20, fader at 0, -10 dBu in)
I quoted the full thing, not just the percentage number. This is referencing the mic input receiving signal at -10 dbu, that equals to 0.2V RMS, with the analog gain (trim) set to 20 db out of a possible 76 db range.
And you're comparing that with, for example, this spec of the ADI-2 Pro FS analog input:
THD+N @ -1 dBFS: -112 dB, 0.00025 %
Again, full quote. That doesn't give almost any of the information that the first spec provided, and can't be compared directly. How do you translate the -1 DBFS here to the -10 DBU above?
Another comparison can also be made between the claimed noise performance of these devices:
ADI-2 Pro FS - Signal to Noise ratio (SNR) @ +4 dBu: 119 dB RMS unweighted, 123 dBA
Sound Devices 888 - Equivalent Input Noise -131 dBV (-129 dBu) max (mic in, A-weighting, 76 dB gain, 150 ohm source impedance)
If we just take the raw number, 129 db sounds more than 123. But does it make for a fair comparison?
Anyway, the point is that looking at spec sheets tells us close to nothing. That's why independent and consistent measuring is important for deep understanding of these products performance. Any large scale assumptions about the state of the industry ("ADCs are far from solved") should rely on solid verified data and not biased speculations. My assessment that headphones amplifiers are a solved issue comes from the many affordable SOTA amplifiers measured on ASR in the last few months – not from reading any of their spec sheets.
ASR does not come close to contain enough information on the state of ADCs at large to deduce any kind of insight.