Yes, your caveat is correct: it's possible that none of the lines present in your oldest commit have survived into the current head commit of your main branch.
Not sure how the article's algorithm deals with renames. If a file being renamed as a deletion and addition, then that conceals the age of the lines.
> Not sure how the article's algorithm deals with renames
It's relying on `git blame`'s default behavior which is: "The origin of lines is automatically followed across whole-file renames (currently there is no option to turn the rename-following off). To follow lines moved from one file to another, or to follow lines that were copied and pasted from another file, etc., see the -C and -M options."
Not sure how the article's algorithm deals with renames. If a file being renamed as a deletion and addition, then that conceals the age of the lines.