It is an outside diameter on the tube. They have a box you lay your tube in, if it is too long it won't fit and if it is too wide it won't fit and if it doesn't fit you can't use it. If you aren't sure take a back up tube that is a little smaller. I get used to a tube and that is the one I want to use also, so practice with both. I would take it, it will probably fit. I say BS, Corey has a fairly unique calling style. He is good! So you have seen one of the best in action, can you keep up with him? I have beaten Corey, but he has won over me more, but it didn't mean anything, I wasn't in the final 5 anyway. And yes I shot a few 3Ds in my time.
Gselkhunter