Yeah, I thought about that too. You're sort of correct but you're also not correct.
I agree that suspending it from a fixed point would be better.
I agree that creating a fixed launch channel for the pendulum would be better.
Having said that, it doesn't matter. My fingers are still enough that I'm not affecting the motion in any significant way. You are assuming that any slight motion in my fingers/hand/arm is sufficient to cause the results to play out the way they did.
It's a valid variable to isolate and control if you're going to be pedantic about it, which is fine, but your assertion that my results are invalid is itself based upon the assumption that your objection is a significant one, and it is not. If my fingers/hand/arm was moving all over the place, I'd give you that, but I clearly held quite still other than the initial swing which is a valid part of the process. After the initial swing, I brought my fingers/hand/arm to a "good faith rest".