If you have no any takedown defense and you weren’t training any kind of grappling sport it’s very hard to stay on your feet. No matter how big or how skilled you are in any stand up martial art without takedown defense you’re going down. It’s been proven many times and this is just one more proof.
Of course when you and your opponent start swinging there is a chance someone will get a lucky shot but percent of a one shot knock down is so small that grappling guy will most likely get on top.
You can also check a guy with no takedown skills having huge problems
Or check some perfect way of street fighting with grappling skills.