electronic configuration - Why apart from when building atoms in the first place, the 3d is the lower energy orbital?

From chemguide.co.uk:

If the phosphorus is going to form $\ce{PCl5}$ it has first to generate 5 unpaired electrons. It does this by promoting one of the electrons in the $\mathrm{3s}$ orbital to the next available higher energy orbital. Which higher energy orbital? It uses one of the $\mathrm{3d}$ orbitals. You might have expected it to use the $\mathrm{4s}$ orbital because this is the orbital that fills before the $\mathrm{3s}$ when atoms are being built from scratch. Not so! Apart from when you are building the atoms in the first place, the $\mathrm{3s}$ always counts as the lower energy orbital."

Could you please explain why this is so?