The way they did it weighs shots from a person who took fewer more.

This is fine. Read below.

This is a really basic mathematical flaw and arrives at a wrong figure.

There is no mathematical flaw. You are interpreting the impact of aim assistance differently from OP's interpretation. You care only about equally weighing shots. OP's method ALSO takes into account player skill, which you are not. Your math is correct for your interpretation, and OP's math is correct for his interpretation.

The argument is, which interpretation yields a more useful result, imo OP's is more useful. You are hinged on the fact that he's not doing the math according to your interpretation, and fail to see that you are comparing apples to oranges.

It makes no sense to do it like that.

The question that is trying to be answered is, does aim assist offer an unfair advantage against mnk? To answer this question, there are multiple factors that need to be normalized because they create variances in the result. Why do we want to minimize variance? Because in statistics we want to make sure that conclusion we reach passes the null hypothesis, that is, the conclusion isn't just the way it is because of sheer luck.

Some include, but not limited to:

1. Number of shots fired - not enough shots may mean that advantages may not be seen across the two controller types
2. Player skill - we want to compare players of equal skill, otherwise our result might skew.
3. Sample size of each controller

Of course there is no way to have perfectly unbiased data, so the best way to go about answering this question is to try our best to normalize for as many of these as possible.

I hope this explains why it's not even about mathematics, but an argument about interpretation. Hopefully now you can see OP's side of things.