Its harder to get steals in the post 1 v 1 because the post player need not worry about other 4 defenders and the spacing is open. Manu might get a steal, he might draw a charge/flop, I don't doubt that. But I think Kawhi would score one him easier than vice versa.
It's kind of an unfair comparison, like pitting a light weight vs a middle weight. Kawhi has at least two inches on Manu ( I don't know the measurements, just basing off what I see), and significantly more muscle mass, combined with the fact that Kawhi is actually a very a efficient post player especially against smaller players (an underrated aspect of his game partly because the Spurs don't feature it much).
Another thing is while Manu was a very good/elite scorer a big part of was his ability to use the P&R. His ability to pass always kept/keeps the defender(s) off balance and guessing. He was still a great 1v1 player, but the threat of his passing and reading defenses and reacting is what made/makes him special. In 1v1 scenario he'd still be great, but not great enough to score on and stop Leonard enough times.