Some people assume that the neck of a guitar should be perfectly straight. However, the neck of a guitar should always have a slight curve. If it were not the case, the strings would buzz. This has to do with the string’s amplitude, which is not the same everywhere. If this difference in amplitude were not taken into account, the strings would buzz against certain areas of the neck.
The photos of the neck and the steel ruler held against it clearly show that light passes through in certain places.
The video clearly demonstrates that in all areas a piece of paper fits between the frets and the steel ruler, except for the first and the last frets.
And that is precisely the intention.