It’s a new engine, which is more than enough to warrant a new title imo. CS has been practically the same game many years and has been incredibly successful all that time. The community does not want major gameplay changes, just minor improvements. CS as a series is not like COD or Battlefield.
CS2 has much more advanced and consistent smokes, vastly improved graphics, more advanced netcode, competitive improvments alongside a new competitive mode similar to FaceIT, and loads of quality of life improvements. I think this is more than enough to be considered a sequel.