No experience at Microsoft but plenty with lawyers in large corps and I would rate another possibility highly, which is that the team wanted to contact him but lawyers recommended "no further contact" as the safest legal IP path. Essentially, they were trying to close the barn door on clean-rooming the software and any further conversation could leak non-open-source ip that would then bring about a liability later on.
>>zmmmmm+(OP)
I can see that happening at Microsoft for sure. There's no doubt legal would've been involved with the conversations and gave heads up to all people on the interview loop.