Sans actually thinks that he believes that - or at least, he hopes that Neil is telling the truth. But he equally doubts that the human will be capable of getting much rest like this, no matter how much good it might do. And Sans can dab at his face from now until sundown, but he can't do anything to help with that.

He can, however, find someone who can do more.

"yeah, well. let's see if we can get you that rest." He reaches out to pat Neil on the shoulder, and then he disappears. And if Neil thinks that Sans' rather fumbling attempts at care are awkward, then his day is about to get even worse.

Because Sans reappears in the gardens, and immediately sets off in search of Grune.