Holbrook is obviously mistaken on that count, as anyone can see by reading the actual warning themselves. Given that he discussed the matter with Milosevic at Dayton and later, there's a possibility he's confused his discussions with the content of the original warning - the Christmas warning says exactly what is says, and nothing about Kosovo being a part of Serbia.
you've actually read the actual warning? Perhaps you can provide a link so that we can read it for ourselves.
Until then, I will take the published word of Holbrooke over your claim!