He says in his letter that the RB17 hypercar is “in final stages”. I expect this is something RB insisted on for his statement, as they’ve been using almost identical language in their press releases on the matter. They want wealthy buyers to know it’s still a Newey joint.
I don’t think an aerodynamics specialist has much to do in the final stages of a production road car, his input would likely have been much earlier in the process.