How does the move order 1. e4 e5 2. Nc3 "rule out Philidor's Defence 2...d6"? So I was reading "An Opening Repertoire for the Positional Player", by Gufeld and Kalinich, and found the title's assertion. And I just can't think why after "1. e4 e5 2. Nc3", d6 would be bad for black.

Also, I find it as a playable option in the Lichess Master database, https://lichess.org/analysis"
While d6 is passive, it is probably not the end of the world there at levels below Master. After Nc3, it is not so much that it "rules out Philidor's Defense" as when white played 2.Nc3, it became a Vienna opening proper.

More typically, when white plays the Vienna, he is looking to attack, and often plays an early f4. With f4 played, you can see how you might want your Bf8 on c5, and after d6, you already ruled that out.