Its arguing that that source code that affects the behaviour of something should be easily discoverable from the point/points its behaviour affects. or alternatively more indirection/obfuscation is worse.

Its not so much about same file, as reducing distance to understanding, whether visually or by some sort of easily traceable path.

Like you would want to init a variable closer to its usage, Or that having a 100 wrapper functions is less understandable than inlining for a single statement, or global mutations are harder to trace then local, and that sometimes its easier to inline a single sql statement then split it out into a different file just because its 2 different languages.

Also, to be clear its possible to write CSS that exhibits less or more LoB. The file thing is more that I don't think HTML, CSS, JS "must" be written as separate files which is what the prevailing best practice used to be, justified as SoC. I just think splitting along the scope/behaviour lines rather than file type is more understandable.