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As the sentence indicates that this relates to the subject of the verb, "for those who code" is correct. Try rearranging this to "they code" which sounds about right, as opposed to "them codes" which is what the implied plural form would suggest would be the replacement sentence, and just sounds plain wrong.
If you can answer who/whom question with "he" then it's "who", if you can answer it with "him" then it's Whom":
Who wrote the code? He wrote the code.
By whom was the code written? It was written by him.
“That which can be asserted without evidence, can be dismissed without evidence.”