Earlier quoted context omitted.
My sarcasm detector is acting a little wonky - there is no real reason for Microsoft to lie about this, it isn't exactly breaking the bank for them.
Microsoft is composed of people. The people we worry about here, are the ones who might be incentivised to discover exploits before an outsider. They would have an incentive to back-date their work, or their subordinate's work.
BTW, as others have mentioned, this is strictly better than the policy of other bug bounties until now, which is "We already found this, so you get nothing"