It's not just about reaching the weight limit, you also need to go below so that you can play with ballast to adjust the balance:
Stella was keen to outline why he wants McLaren to get under the limit as soon as possible.
"Certainly, working on reducing the weight of the car is always a development item, because even if you are at the weight limit, you want to be actually under the weight limit, because then you can play with ballast," he explained to media, including RacingNews365, during testing in Bahrain.
https://racingnews365.com/mclaren-hopin ... ttleground
So Red Bull's "project" is not just 15-20kg, it's closer to 30kg to get the car how it should be in terms of the min weight, and the room they need to fine tune the balance of the car.