Yeah, there's not much you can do about that. It's frustrating.

But, really your only job is give them confidence that you can get the job done for them. If they pin you down like that but you can speak about your abilities with confidence, you can say, "I realize this wasn't in an official capacity, but I'm confident in my current abilities and my ability to pick up anything I'll need to get the job done. I won't let you down."

If they don't buy it, that sucks, but ok. Give yourself a fighting chance. Persistence is a skill and you can demonstrate it in the interview.

"Now, if you know what you’re worth, then go out and get what you’re worth. But you gotta be willing to take the hits, and not pointing fingers saying you ain’t where you wanna be because of him, or her, or anybody. Cowards do that and that ain’t you. You’re better than that."