"It's fine," Ritsuka assures him. He's not the first Servant to choose to conceal his name here, just the first she doesn't already know. And to be perfectly fair, he has had a tendency to accidentally nearly blurt out Sanson's name without thinking on a few occasions...

"Anyone could have picked up a knife or something else to use up as a weapon from the kitchen." So that doesn't really narrow it down much yet. "But to use poison they'd have to actually handle the food at some point, and know who was going to eat it."