Pretty sure he was simply surprised that Goku and Piccolo could do it.
You need to realize that concentrating one's ki is not a common thing among the universe. I'd wager the majority if not all the people Raditz has fought and killed for Freeza didn't even utilize ki very often much less concentrate it. Hell, even Freeza's numerous foot soldiers had lil energy blast guns. I feel like only a select few plus Zarbon, Dodoria, the GF and Freeza himself were the only ones that knew how to concentrate blasts aside from possibly Raditz and Nappa and Vegeta clearly could, Vegeta's Galick Gun is in all likelihood concentrated.
Even if Raditz couldn't himself, he was just surprised his "lowely" brother could even after failing his mission and all that bullshit.