Harry chose to believe Snape was acknowledging his efforts and helping to bring the conversation back to what he considered the most pressing issue. Because his other choice was to call the man a black-hearted bastard, and Harry didn't think that would solidify his leadership with the Order too well.
"I traced the zombie magic back to their graves, Snape, and couldn't go any further. Are you volunteering to investigate the magical trails?" he asked with a challenge in his tone.