Intelligence agencies are very careful about sharing their findings, even with "friends", because the findings will disclose some information about their capabilities and possibly methods.

Let's say agency A has some scanning capability on open source software that detected this backdoor attempt by agency B. If they had gone public, agency B now knows they have this ability. So agency B will adjust their ways the next time and the scanning capability becomes less useful. While if agency A had told Microsoft to "find" this by accident, nobody would know about their scanning capability. And the next attempt by agency B would only try to avoid having the performance impact this first attempt had, probably leaving it visible to agency A.