It tries to target the retarded 120Ω standard which nobody ever cared about. In reality, the lower the output impedance, the better.
2.5W into 4Ω is about 3Vrms. Some larger headphones could basically run from this directly, no resistors whatsoever.
If you want to push it down, say 1.5Vrms, then you need two equal resistors such that their sum is about 4Ω (if you disconnect the speaker) or several times higher than 4Ω (if you don't). That's all, no R3 necessary at all.
For 1Vrms, same thing but 2:1 ratio resistors. And so on.