What I think I was pointing out earlier was that they mention this round-about method of producing these conjugations for negative imperative, rather than simply saying that it shared them with present subjunctive, was that at the point in learning where people would first encounter them, just saying to use the subjunctive form would have no meaning. As for the round-about method of learning it, I think it's as @Cifi said. I believe it's done that way because quite a few irregular verbs use yo form stem-changes, with the exception of a few like 'comenzar' (and even that includes its yo form stem-change, just with another one added) and some other irregulars like 'dar' and 'ir'.
So we have our verb - let's say 'cerrar', and we need to conjugate it so we can say "Don't close that door!".
What's said at this point is thus, "Take the yo form (so 'cierro'), drop the -o and add the needed ending ('cierres'), and finally check if any additional stem-changes are needed (none, still 'cierres')".
You'll end up with "¡No cierres esa puerta!" once you've translated the other words.
The problem with it, of course, comes with those irregular verbs whose present indicative yo forms don't end in -o (like 'dar', 'ir', 'ser' etc.) so a better course would be 'Make whatever the needed stem-change is, and switch the endings' - the problem is memorising those stem-changes first.