You did not carefully read the article you yourself linked to. Dollar cost averaging is a generalized concept. What the author compares is a full-sized investment or time-specific partial investments. So, dca is a concept and you draw conclusions from one single approach to dca. There is no mathematical proof that dca works or not because it is one single concept that is only part of the overall investment process. Its as if I tell you that getting higher education does not pay off because those who undergo it and become philosophers do not recoup their investment. I did not include those who become lawyers, doctors, financiers,...the same applies to this article.
What if I told you that dca hugely outperforms a lump-sum investment approach if instead of investing the parts at time-shifted periods you instead disregards time and buy companies with great fundamentals but depressed stock prices because of overall market sentiment. Each time the stock price trades another 2% below your previous investment you invest another portion. It is one alternative approach from many but it clearly demonstrates that you are comparing apples and oranges here.
Another example to debunk the point made in the link is if I claimed trend following strategies do not work because the market is 70% of times range-bound and the strategy cannot make money in a range bound environment thus it has to be inferior to a range-bound strategy approach.
Hope you got my point. So, in short, there is no mathematical proof. I could specify a strategy approach that hugely under-performs when investing in a dca way and another strategy that hugely outperforms a lump sum investment approach.