A good thing if it insulates defence spending from being robbed to pay for other security spending in the current "fiscally neutral" review. But presumably unhelpful if the Chancellor sees it the same way as the press - ie where any increase in defence funding would be a victory for Williamson and a defeat for him.
And of course Spreadsheet Phil knows the MoD better than most and will have a good idea of just how efficient or otherwise it is. On the positive side, with defence being unusually high on many Tory mps' list of concerns, May won't want to see a Williamson resignation so he may have a certain amount of leverage there. Unsatisfactory fudge may be the most likely outcome, with high profile items saved and continued trimming elsewhere in what they hope will be an under-the-radar sort of way.