I don't think it's a choke or at least intended to be a sustained choke due to the impact effect drawn around where Batman's head crashed against the surface of the ledge.
If Slade was indeed choking Batman, it likely lasted all but a moment as he was continuing talking almost conversationally like in between strikes, which seemed to be one right after another.
No doubt an impressive feat, but not really indicative of strength in of itself. They did have an explicit test of strength before that exchange of blows though, iirc, and judging by Batman's grimace and the lines of motion, I'm inclined to say that Slade held the advantage.