does it really matter though?
Trying to be precise in language allows you to communicate an idea the most effective way, imo, and at the end of the day that's you're goal. At least it should be in technical writing. If the reader has to pause when they see "codes" instead of "code" because it's just a bit off then that is a tiny failing in your communication.
Definitely not the end of the world, definitely not something to be an asshole about, but it is something I would maybe politely correct if I knew the person well enough.
Also don't take this for me saying I'm perfect in my written communication. I still don't know how to use commas and I've had them explained to me from all grammatical standpoints multiple times. Trying is all you can do.
I had thought "a code" and then "codes" may have come down from the punch card era of programming where as you said it would be more of single routine for a specific computation which I guess then would have filtered down into scientific computing as programming evolved through fortran and others and now it seems it's all merging back together.