Let's not exaggerate here, Lewis didn't have a terrible jab. He had a very good jab, but I will say it was overrated, as you say Bruno out jabbed him, Mercer was able to jab with him and Vitali also out jabbed him early on before Lewis adapted. Whatever you think of Lewis as a trainer or of his jab what he was trying to show Wilder had merit, though I don't think Lewis communicated what he was trying to show Wilder very well, which is why he'll never be a great trainer. Lewis was showing Wilder how he could throw a jab with his arm extended. By extending the arm you can you it as a range finder and keep your opponent at arms length and when required fire off a quick jab to keep your opponent under control should he try to step inside the extended arm. It clearly worked very well of Lewis during most of his fights, though I still think his best jabbing performance was against Rahman in the rematch where he threw it piston like rather than the pawing jab we all too often saw. The pawing jab has it's place but Lewis over used it too my liking.
He is teaching him good things...but sometimes ability to execute is more important than the form. His elbow is still abit out in this video...but i'm not 6'7" 240lb's. No two fighters are alike and will throw punches with subtle differences. Windmill Wilder bless his cotton socks. Spilka was beating Wilder to the punch for the first 5 rounds whilst wilder missed & fell short.