Or, you know, Cowl simply absorbed/created necromantic energy, or used it in a metyhod similar to what Kumori did to prolong that one guy's life. It's not simply a matter of "did he raise zombies or not".
Remember, Harry channeled a massive amount of earth energy in Changes, by tapping into a ley line. Cowl could've simply channeled through a necromantic leyline, and called it a day. Or he could've eaten some shades.
Just saying, its difficult to just automatically assume he used some necromancy