# Transition metals have a half filled s band in their metallic states

I was reading this "Chemical bonding at surfaces and interfaces" book. Here in page 69, they argued that interactions of transition metals to s band of adsorbate would be same because transition metals have half filled s band at their metallic state. My confusion is in electronic arrangement of transition metals I am seeing $s^2$ except for atoms having $d^5$ or $d^{10}$. Even $d^{10}$ is actually d block metals not transition metals. What could be the possible explanation of this line?