"The front of your mouth plays a vital role when pronouncing consonant n and l ". instead of generalizing it to "English".
Listen to the examples he gives closely, "not", "lot". When pronouncing "n" and "l", of course, the front of your mouth, actualy, tougue, makes the difference. But as soon as "o" kicks in, it obeys everything you have learned for pronouncing a correct "o". It doesn't entirely rely on the "front of your mouth" anymore.
Don't get confused, it's his statement that is a little misleading.
