I wouldn't doubt it came from someone at AMD originally, but if it's someone working on the PS5 semi custom version, they wouldn't and shouldn't know if XB is using it as well. That would explain why they think it's being worked on for PS specifically. The AMD teams would need to be separate so that MS and PS wouldn't have to worry about custom info getting leaked directly to the other group.
AMD would also know when PS and MS plan to launch their next gen hardware, since they are working on it, so if PS and MS want certain general things, it would only make sense to incorporate them into the Navi design as much as reasonably possible. If it was just a random AMD employee, since PS is the market leader by far, that may be why they didn't mention MS.
There are also rumored to be 2 versions of Navi. The low to mid-high end cards are supposed to be Navi 10, which should be launching mid to holidays this year, and the mid-high to high end Navi 20 version launching sometime in 2020, an entire year later possibly. Navi 10 may be designed more towards what PS wants and Navi 20 towards what MS wants. Both being very similar, but having their own required differences.
If I had to guess, just like how PS4 is still vastly outselling XB1 at a much higher price point, PS5 will be a little bit weaker than XB Anaconda but will be the same price. Based on the specs we've learned about so far, unless the RAM and GPU performance is held back more than some would hope, I'd imagine both next gen consoles are going to land at $499. I wouldn't be surprised if PS5 drops the price $100 after the first year though, while Anaconda remains at $499, especially if Lockhart exists at say $299.