I don't think his passing is "bad", it's just not that good. Considering his defensive struggles, you need to pair him with a strong DFD, and puck moving DFD aren't exactly common. My biggest problem with Oleksiak is that he plays way softer than he should, Johnson is way more physical than Oleksiak is despite Oleksiak having 5 inches and 30 lbs on Johnson. If Oleksiak had Johnson's physicality, he'd be an absolute monster and very few people would be complaining about him.
My concern is that Gonchar isn't the type of coach that will teach him how to be a physical defenseman, he'll be the kind of coach that turns him into a PMD instead of building on his strengths. You'll be left with what Oleksiak is right now, a guy who has okay puck moving talents and doesn't use his massive size. I would rather see Oleksiak be a worse puck mover and a physically imposing defenseman, I don't think he'll become that though.
Pettersson doesn't currently have the body to be an effective physical defenseman - so Gonchar teaching him to be a PMD is the best possible thing for the team and for Pettersson.