I think that's a myth.

Microsoft used to (and some teams probably still do) use an internal tool called Source Depot, which definitely was a fork of Perforce. I've seen this confirmed in a couple of MS blogs, and, anecdotally, I've used both.

I can't find any evidence that TFS is derived from that codebase.

However, I can believe that TFS uses a bunch of paradigms from Source Depot (and hence from Perforce), because that's what the developer division at Microsoft were used to.