A line is a unit of change that git can report on.

If it's a separate file that is scoped to some specific concern, sure. But its tgat grouping by concern that is key. Not separation into another file. Extracting ra dom bits of code into separate files would be _worse_.

> A line is a unit of change that git can report on.

Yes and no.

Git doesn't store lines, it stores files. Git diff knows how to spit out line changes by comparing files.

So to run git blame on a 10k line file you're reading multiple versions of that 10k file and comparing. It's slow. Worse still is that trying to split said file up while trying to preserve history won't make the git blame any faster.

Yes and yes. While agree with the general points, note that they didn't say "unit that git stores", but "unit git can report on". Git can totally report on lines as a unit of change.

got diff understands function boundaries, and for many languages will “report” equally well on a single file.

It’s a good idea to break things down to files along logical boundaries. But got reporting isn’t a reason.

Git diff absolutely does not understand function boundaries, it's diff algorithms routinely confuse things like adding a single new function, thinking that the diff should begin with a "}", instead of a function definition.