Actually, the other way around really. Someone looked up the musical information and measured and found a discrepancy. Given that most western music does in fact use 12TET, the claimed "out of tune-ness" was reasonable to note. The problem is that there are cultures and genres of music that do not use 12TET. In the west these are rare, and many musicians would not know about this. But the measurer had the misfortune to be measuring music that did, and thus was incorrect.

I think Just intonation, or an approximation to that, is what people are instinctively using, unless they are consciously tuning themselves to a piano.

It sounds much more "in tune" to the ear, because of the nice ratios.

It really wouldn't surprise me that raw recordings of many singers are in fact more just than equal temperament.

This makes sense, but it would imply that they have "instinctively solved" the fundamental problems with modulation and harmony that just intonation creates. This seems unlikely.

You might find the book "How Equal Temperament Ruined Harmony" interesting, which presents a reasonably strong case that pre-1900s western music treated enharmonics as distinct pitches ("enharmonic" in the old sense seems to have referred to the distinction). Singers knew about major and minor semitones, and they knew where to place commas. Though, no one seemed to go as far as doing full just intonation due to various kinds of obvious and non-obvious impracticalities.

All in all, I think it's safe to say that string players and singers prefer slightly flatter thirds in harmonies than 12-EDO -- which is what the above amounts to -- and deal with commas accordingly.

Realistically, they probably just sing an approximation that's close to JI while unconsciously allowing the pitch to drift a little bit so that comma pumps aren't a problem.

That seems more plausible than that people naturally sing in an artificial tuning based on repeated multiplication of the twelvth root of two, even if that's the musical context that people are most exposed to these days. Those whole-number ratios are just too good to resist, even if a singer isn't fully aware of what they're doing.

I think you have a misconception, western singers use "non 12TET" very well

Perhaps you could name some of the well known pieces in the western canon since the advent of 12TET that are written for non-12TET tuning/scale systems. Certainly there are folk traditions that veer closer to non-12TET but there's very little music for voice and instrument in western culture that isn't 12TET because so many of the instruments are tuned to 12TET.

Barbershop quartet is the style that comes to mind where it's used all the time

I did not know that - thanks very much for the enlightenment!

My wife and 1 daughter both sing in acapella groups, but their various different groups over the years have never done anything in JI, although my wife also sings Balkan songs which are whole other story of course.

I don't think that would change anything.

Maybe a portion of university-trained musicians might know it from their music history classes (or maybe they're Adam Neely viewers), but I believe most professional and amateur musicians have no idea about just intonation or equal temperament.