Luke's pretty sure that sharing this information won't be dangerous, and if it is? Well... he'll worry about it later.

"The good news is the Empire's new planet-destroying mobile battle station was destroyed."

His delivery is meant to tall Ezra exactly what the bad news is. It's... look, it's not like Luke knew anyone on Alderaan, but it's still pretty sobering thinking about all the loss. Honestly, he's probably lucky that he still can't feel the Force the way Ben could, because that would have been awful to experience. No thanks.