Try a smaller tip and/or flame. The trick is to control the preheat so that both the tube and the bit are up to temp at the same time. You will want to continue to adjusting the temp as you apply the filler. In the vid the flame stops being moved about as the filler is applied, it takes practice to be able to move both hands in somewhat different paths. It's easier for a beginner to practice on somewhat thicker walled tubes as the heat up rate will be slower.

BTW do you know the trick of getting a tube that's 1/8" larger in diameter and of a .058" wall to act as a sleeve or lug? As you develop comfort in brazing the small bits consider trying to flow the filler through a sleeve/lug. Andy