MVDP gets to do that because he has the green light to do that. Nothing happens without the approval of the DS. If you ignore the DS, you are putting your career in peril.
Milan-Sanremo is a one-day race. MVDP is built for one-day races. He will be leading out Phillipsen at the Grand Tours. However, Phillipsen is scheduled for Milan-Sanremo. Will he have an unconditional green light for that race? Probably not. Conditional, based on the race situation? Most likely.