Correct. A pitcher can stride out as far as they want and land as far foward as they can (assuming everything else is legal with respect to where they're pushing off, leaping, dragging etc.).
The circle really applies to one rule and one rule only- the Look Back Rule.
80 m.p.h doesn't surprise me. I play with and against several pitchers that have been clocked just a notch below that. One fellow has reached 77 and his pivot foot barely leaves the ground. Maybe if he jumped like this guy he would gain that extra three miles per hour!
Not to knock girl's or women's fastpitch (I love girl's fastpitch, my daughter played girl's fastpitch, I coached girl's fastpitch and I umpire girl's fastpitch) but there's no getting around the fact that adult men will be bigger and stronger. After I umpire a men's fastpitch game, it takes some adjustment when I go back to doing a girl's game. Even the fastest female pitchers will seem like they're in slow motion!
Now that you said that, i recall having that discussion a few years ago and we never found a rule stating you had to stay in the circle on the pitch. Funny how that hasn't come up in a long time and if i had seen a girl do it, i would of questioned it too.
This guy would be spooky, he has to be landing somewhere at least 12 feet from the mound. He also has to be one tired dude after doing that for a full game!