Hard for me to say that's a dirty hit. He's low, but not LOW. The contact initiates just above the knee, not at/below it. If Godwin's foot doesn't get stuck in the ground that comes away looking like a clean, solid hit, and a routine play.
Ideally every hit occurs between the hips and shoulders but ideal situations are tough to come by. From a long-term health perspective, preserving the brain is absolutely the best decision. From a short-term/career earnings perspective, protecting the knees/ankles is of more importance.
A truly dirty hit is someone launching themselves head-first repeatedly (see Sendejo), or deliberately lunging at ankles/knees, or coming in extremely late. That was a questionable hit, but I'm not sure if it was black-and-white dirty.