It’s not about adapting to the car per se, it’s about being able to set it up to the driver’s driving style. Reading what Andrea Stella says about Alonso, he doesn’t just drive whatever you give him. He’s one of the best at getting a feel for the car fast, and giving his feedback effectively, which helps the team a lot in setting his car to what he wants. That’s how he “adapts”.

From Newey’s book you can also extract that Vettel is also very detailed in his setups and in his feedback, in a contrast with Webber or Hakkinen, according to him. Reading between the lines from Vettel’s declarations, he doesn’t understand the car and can’t set it up. He knows the car he wants, and knows what the car they built is like, but can’t match the two. Wether that is because he doesn’t have the data necessary (could happen since the team won’t be very likely to give data on their updates to a driver that is leaving for a competitor) or because the car is a big departure from the car they had in 2018 (very likely, since they had to change their philosophy to accommodate an engine they didn’t plan for) we don’t know.

A lot of people here want it to be an easy explanation. Vettel only likes a rear planted car and can’t adapt like Leclerc. Ferrari gave hum a tractor. But it’s more nuanced than that and these two easy explanations are always given by the guys who dislike him or like him too much.