There is something to that - Microsoft has been spending a lot of money and resources trying to encourage Silverlight adoption. One way to do that might be to buy the rights to Flash.
Microsoft already has a problem with runtime fragmentation - their software and dev platforms are split between native and COM-based stuff (Windows, Office, most of their consumer apps), .NET/CLR based stuff (.NET, Silverlight, XNA, most of their enterprise apps) and increasingly JavaScript/web based stuff (IE, most of their web properties). The last thing they need is to add a fourth runtime to those.