An official source would be a press release, or otherwise from Microsoft themselves. If Microsoft is wanting the RTM signoff to be kept confidential at first, then it behooves any employees making statements to the press to want their identities kept anonymous when they are cited by reputable journalists such as Tom Warren or Brad Sams neither of which have a reputation for spreading false information. And it is a journalist's obligation to respect the wishes of sources to remain anonymous especially when exposing them can result in termination of employment for violating a Non-Disclosure Agreement, and/or criminal liability and/or civil liability. When concerning anonymous sources, a journalist only needs to verify the information from at least one other additional source before they report on it. If either Brad or Tom simply ran with an article before redundant verification, they would have been laughed out of any reputable technology organization a long time ago. They are not in the habit of selling woof tickets.
That's just the thing, tho. They didn't claim to have a MS source. It's very likely they are referencing the PCBeta stuff that FaiKee mentioned. Edit: I've asked Tom Warren on twitter. I have no idea if he'll answer me.
It does to me. "Sources familiar with X" is standard journalist terminology for "an internal source who does not wish to be identified." Happens with political reporting all the time.