If Kou notices Kira's slight slipup (and he does), he doesn't mention it. He may have seen some of Kira's past thanks to Maruki, but he won't bring it up if Kira won't.

Instead, he just accepts his coffee, looking down at it and- yeah, he doesn't know what he expected.

"Very mature," he deadpans, fixing Kira with a look as he lifts the coffee...

... and takes a sip, still staring straight in Kira's eyes. He hums, some foam clinging to his upper lip as he puts it back down, letting the flavor sit in his mouth.

"Acceptable," he says, which might as well be Kou-speak for 'that was delicious'.