Of course Frisk isn't telling the truth. When do they go easy on themself? Themselves, honestly, they all refuse to. (The thought that he's occasionally a hypocrite doesn't occur to him.)

[I won't be offended, believe me. It's a little disturbing even to myself. But it is--my new normal and I'm incapable of feeling pain.]

Which begs the question of how he knows he doesn't feel pain. Oops.