Technically, Buzz does not gain I-frames while using Laser Action. his ult causes him to move. Any ults or skills which cause the user to move for an extended period of time simply make them untargetable and immune to any skills that would normally interrupt the skill. (Can anyone confirm?) It is easiest to tell this against Damage over Time heroes, as the DoT effect is already handed out, they still suffer damage mid-skill and still can die before it finishes.
But lets get back on topic. Can you post another video to analyze