I think it was a bit of an early stoppage but I also sympathize with Herb and know it's a completely different experience watching the fight on tv versus reffing it up close and personal. Heck there's a chance he didn't even see it was a knee that dropped McMann. Maybe he just saw them scrapping and Sara crumple. Who knows?
Depending on the scenario for refs deciding a stoppage they can be "damned if they do and damned if they don't."