I wouldn't make that comparison at all. I would have guessed that his defensive stats are a lot better, but in general his stat profile tells the story in my opinion:
The middle section is outright pathetic for a supposed €60m winger. He doesn't really participate in his team's build up, he doesn't really carry the ball or dribble a lot, his main strength in that bracket is receiving the ball in high areas of the pitch (after the opposition exposed itself on a counter attack or Wirtz did his thing).
What he has going for him this year is his finishing, which has been pretty great (though for the first time in his career), his xA is good as well, but I wonder how much of that is because of Leverkusen's counter attacking style, where you can milk a lot of xA by just squaring it inside the box. And I wonder how much of it will remain now that Wirtz isn't there to run the attack for his team.
Dembele on the other hand is indeed erratic, but he's a player that can run his team and when his high risk/high reward moves pay off he does stuff that a lot of other players, including Diaby, can only dream of.