Whether inside or outside of the guitar, a camera would be able to see the same things. This is a modified video. The vibrations would be so fast, the eye (and I seriously doubt an I4 camera) could pick them up.
That’s a video artifact called aliasing. It’s related to the effect you sometimes see in older films where a wagon wheel will appear to rotate backwards. The reason it’s noticeable here is that the dark strings are starkly silhouetted against the bright sky. That’s not the case normally. I don’t think this video has been doctored.
It actually is not modified. I thought you might be correct. . . and so did my daughter. But when i thought about it, I realized that it would not be. I was wondering the same thing. . . but it doesn’t take the vibrations to be exactly in time with the 30 frame per second video. . . Just that they be in tuned to some equal multiples of that. . . and then it would work fine. For example a 60 cycle hum would show quite nicely. . . so would a 90. . . 120, 150, 180, etc. For example a natural A is 440 cycles per second. That would not be seen on the video. . . but detune it up by just 10 cycles to A 450 (divisible by 30) and you’d see standing waves on the string. A 440 would be visible with a moving wave running backwards at ten waves per second.
Then I found the guy who put his phone inside the guitar had not just done it blind. He HAD videoed it from outside and got some similar waves up and down the strings. . . and ALSO used a violin bow on it. Then he videoed a violin in a similar way. . . and got the same kind of waves only much more clear. Here is that video.
I would assume that the phone did not vibrate exactly with the individual frequency of the sound, but would also have some induced sideband and other physical factors to allow for the effects shown.
That, and you don’t get sine wave looking patterns in the string.
Ever...
Clever editing and a few good Photoshopping skills.