At first, Sonic is thrown by the no. Why would Add say no? He's not going to eat the entire cake by himself, that's ridiculous. But with the way Add is watching him, and with the way Add is talking, it's clear that he's being tested.

Or provoked.

Either way, Sonic doesn't feel any guilt for swapping half of the cake for half of his mochi with lightning fast speed.

"Be that as it may," he says, pausing to shovel a forkful of cake into his mouth, "I'm the god of thieves, and I can always simply take what I want."