Hey, this is an internet forum. It’s no fun if we have to back up our statements with authoritative sources! [;)]
This is the best I could do. For METARs and TAFs being in true, Chapter 12 of the Pilots Guide to Aeronautical Knowledge (an official FAA publication) says about METARs: “The first three digits indicate the direction the true wind is blowing in tens of degrees.” and about TAFs: "The first three indicate the direction of the wind in reference to true north." (my emphasis).
As for ATIS being in magnetic, section 4-1-13 of the AIM, says “ATIS information includes the time of the latest weather sequence, ceiling, visibility, obstructions to visibility, temperature, dew point (if available), wind direction (magnetic), and velocity, altimeter, other pertinent remarks, instrument approach and runway in use.” Section 7-1-2 (b) of the AIM describes AWOS and there’s a note that says "Wind direction broadcast over FAA radios is in reference to magnetic north."
Additionally, section 4-2 of the AIM, called “Radio Communications Phraseology and Technique” has a section, 4-2-10, which says “The three digits of bearing, course, heading, or wind direction should always be magnetic. The word “true” must be added when it applies.”
So I think that, in general, what I said was true - if you hear it on the radio it’s magnetic and if you read it on paper, it’s true. But it’s more of a “rule of thumb”, or an easy way to remember things, than some iron-clad guarantee. I’m sure if you searched hard enough, you could find SOME kind of written weather product in magnetic direction; I’m mainly talking about the commonly used ones.