There's no "parser" that needs to be "smart". It's just a PHP file that runs as part of WordPress loading. The reason you need to put edits before that line is because the line that comes after it is this:
require_once( ABSPATH . 'wp-settings.php' );
And this line basically loads all of WordPress. So any definitions added to wp-config.php after that line will be useless because WordPress has already finished loading.
You could conceivably add code between the comment from your question and this line, but the existing lines that are already there are checking for the existence of the
ABSPATH constant, which means that if you wanted to redefine
ABSPATH yourself, you need to do that before the "That's all" line.
So instead of saying something silly like "Define ABSPATH before this line, but it's ok to make other changes after this line, just make sure not to make them after the last line", it just suggests making all edits above those lines.
There's no good reason to ignore this comment.