I'm no pro musician, as he is, though I do dabble. I can detect the out-of-tune chords, but he can tell which string is out of tune on the guitar. To me, that's just amazing.
But, here, I'm not sure what he's saying in this video. On the one hand, everyone he talks to about his son's "A" string being out of tune is saying: How would I know, I've never heard the song before. Then, he ends with all of us being inundated with precisely tuned (and precisely timed) music these days. Given that, I would guess people are more sensitive to slight out-of-tune with itself instruments?