There are a lot of things out there that people think or teach that are incorrect. There are many about ice and frost. The instructor you had was correct about trying to remove the frost but he though that clear ice was safe. That's what I'm talking about. Any amount of frost or ice on the wing is going to reduce lift and increase the weight of the aircraft. Doesn't matter if it is clear, rime, mixed or whatever the case may be. There are many other misconceptions too but I won't go into detail, I think you get the picture now.