Magic is just steroids to Tirek. It Enhances his size and raw destructive power but thats it. He stole Discords chaos magic and all it could give him was a bigger pump.
I don’t actually think this is a continuity error. I think it was more like he had access to Discords magic in the season 4 finale but never actually used it. IIRC he did nothing chaotic or random with his magic in those episodes.