1. BS. Because he didn't cover the left side against Mihawk air slash doesn't mean he can't and i honestly don't see why he couldn't. When he hit Aokiji or Crocodile, his left side was covered in diamond for example1. The area around Jozu's left shoulder cant be covered in diamond. Sabo can just aim for that area.
2. What makes you think Jozu has better Haki? Jozu's physical strength is one of the best in the series, and then he also has the Diamond DF which makes his punches even stronger and gives him some very good defense, and those two things are some big reasons why people decide to learn CoA. All Jozu would really need CoA for is the ability to hit Logias. Meanwhile, Sabo had no DF until like 30 min ago and was still strong enough to become the RA's no.2, so he definitely has some great Haki. There is no evidence that Jozu's Haki is > Sabo's Haki.
3. All Jozu did was manage to hit a distracted Aokiji, and a few seconds later Aokiji returned the favor. His fight against an admiral wasn't any more impressive than Sabo's.
2. It's because he managed to make one of the logia trio bleed something even Whitebeard was struggling to do. He has the greatest physical strength feat in this manga yet and physical fighters like him or Garp have the tendancy to have great armament haki(Garp was the only one who managed to damage Marco in his phoenix form, something Kizaru was failing to do). Sabo on the other side can't even push an admiral back
3. It was more impressive because he was evenly fighting a serious admiral when Sabo was struggling against an admiral not going all out and was only using his opponent as an excuse to stall