He’d call out the chord he was fixin’ to play and then go to it and it looked weird to me, so that’s why he said that. 
Q
I’m a lefty who plays righty, although I can play the other way too.
I’m pretty much truly ambidextrous - can write legibly and throw a ball equally well with either hand.
I actually attribute my ambidexterity to two things - being forced to play right handed - baseball and guitar - along with years of piano lessons at a young age.
I play and teach a bunch of different instruments - some left and some right. The weirdest is accordion, which is easier for me to play backwards for some reason. The rest, I’m stronger at right handed, but my natural instinct for things like pen or fork is still left.
As a minor nitpick, Hendrix would restring his guitar for left-handed playing, while Cotton would indeed play the guitar “upside-down”.
Please carry on.
just to nitpick your nitpick, Hendrix’s guitar was indeed upside down, although you are correct in stating it was restrung opposite.
Mark Knopfler is another leftie who plays guitars right-handed.
If I might chime in…
If someone is playing in standard tuning and the song doesn’t require a ton of fast chord changes, I can usually follow them by sight and provide a decent rhythm accompaniment. After the first verse or so, it gets easier from repetition.
yes, this is one of the things that gets significantly easier once you’ve been in a band for a while…you just kind of learn to read what’s happening.
Gets a lot harder in jazz, but it’s still possible. Really in all cases it’s more of a case of visual cues + some degree of ear training.
I don’t necessarily know that the guitarist just moved from a Bmi7b5 to an E7b9 to Am9 to Bb7sus, but a little theory knowledge and ear training makes it pretty obvious, and certain changes (4>1, 5>4, etc) are easy to see…
It’s a combination of all the cues,and filling in the gaps.
Live and learn. I always take a guitar player with me out on the field trips – seriously, I always like to have a guitar player, even before hiring a drummer or deciding there’s not enough pay for more than a duo – and even though I usually end up zoning out on their fret hand, I never really figured anything out about what they’re doing visually. Ear works, though (I find)!
Of course, they can always just look at my LH in a keys/guitar duo and get what I’m doing, and since I usually end up leading or picking tunes, that works fine for me. Make them work for their supper, and then they can go cruise for cougars. Or, more likely, vice versa, ahem. Bastards hogging up all the glory, making me turn up! I kid.
I wonder what I’m missing to have not bothered to see what specific chords they’re at. I am pretty nearsighted without glasses, which I don’t usually wear, except for prescript sunglasses outside, so that might be it.