I see a juggler handling an unspecified quantity of balls. They will not tell anyone how many balls are being handled. How do I know what they accomplished? How would I replicate it without knowing?

You've proven that the juggler's instructions alone are insufficient to juggle five balls. In this case, the juggler presumably has muscle memory you don't, which is equivalent to running an experiment on equipment that behaves differently. Or perhaps the juggler didn't convey the special trick you need for four or more balls.

Maybe the juggler was juggling 5 balls, but they made a mistake with serendipitous consequences and then told everyone they juggled 5 balls, when they really don’t know how they were juggling 5 balls. In fact one of them might have been a different sized ball or maybe it was a cube.

Worse still, they won’t let you check their original balls to see if they actually are capable of being juggled.

Lol ball juggling /s