Between design diaries, excerpts and the DDXP characters (and even the very first spined devil stats from last year) it is clear that stat mod is exactly the same as it has always been, and that you only add stat mod to damage.
Attacks, defences and skills benefit from a "+1/2 level" bonus, and monsters happen to have that included next to their ability scores because most of the time you are wanting an ersatz (unskilled) skill check from the monster rather than its ability mod.
Cheers