HP not being a software powerhouse with years of success can at best seem to imply it might drop Windows on its PCs. That is, if it is not given a cash gift from some outside benefactor.
Even if it does not work it has lost nothing by allowing sales of its OS on a small fraction of the PCs it still sells, i.e. to those that do not insist on having those must have MS applications and games that run only on its OS. So, at worse, for MS it is bled a bit more than it might otherwise, however, if they buy off HP their cash flow from one of their few cash cows has been lessened.
For HP the risks are minimal either way, if their OS sells into the corporate market they lessen the clout of MS. Or if they are bought off, they never promised anything to their customers other than a tablet and some phones. However, their cash flow is enhanced or they can undersell some of the competition. Either way they gain.