Just leave it alone, if they put stitches in a spot thats wet and moist it can harbor infection, thats why they didnt do it most likely.
It will heal in time without stitches. I've bitten my tongue through and my lip and have no scars (no stitches). Just be careful eating salty foods - that may sting a bit!!
The ER physician's opinion seems reasonable. The injury should heal on it's own without stitches. You need a clean wound line, in any event, for stitches to be useful.