So he progressed offensively...by a lot.
As someone who watched each London game sans two last season, I vouch for 100% of Starry Knight's commentary. In fact, if one peruses the London thread inhabited by fans who missed very few games last year, there hasn't been a single dissenter from his opinion.
You are right that Zadorov's offensive numbers improved (doubled). But his icetime skyrocketed out of the team's necessity in rolling 5 D and playing 27-30 MPG. The increased production is partially attributable to the increased ice time and offensive zone starts relative to the prior season. Starry Knight (or any Knights fan) won't refute Zadorov's offensive progression, but will note that the stats overstate that progression to an extent. But the much larger emphasis Starry Knight is making, and Knights fans here universally agree with this, is Zadorov's defensive game stagnated and his compete level regressed. This was highlighted during the Edmonton/Guelph Memorial Cup games (they're available for purchase on Vimeo if you couldn't catch them). There were 4 goals in those 2 games that were primarily his fault. One of the Bertolucci goals comes to mind where Z was plain outhustled to the net by Bertolucci who was on the sideboards (Z was inside the faceoff dot) enabling Bertolucci to snipe a goal uncontested when the puck came to him while Z was solely accountable for him on the play. On all 4 goals, it came down to heart, and Z was beat. I won't inject some of the off-ice rumblings regarding his focus, but if true -- they weren't pretty. And I was far from the only fan to hear rumblings.
If I was ranking last year's Knights by heart/compete level (irrespective of ability), I would rank Zadorov dead last. I would also wager the vast majority of Knights fans would rank Zadorov dead last too. Ask this question on the Knights thread in the OHL forum, and I would be shocked if Zadorov didn't accrue at least 90% of the votes.
Ability-wise, I think Z can develop into a top pairing defenceman in the NHL. But he'll need a heart transplant (metaphorically-speaking, not literally). It's entirely possible playing in the NHL rather than OHL can do that for him, but only Z knows the answer.