The math itself does not need any reference, if it is trivial enough for lay people to follow, which I think is the case in the answer you mention. Though in most cases I would at least expect a Wikipedia link for the specific formula used if it is something beyond basic high school mathematics. In this case I suspect that there are established formulas for population genetics that one could use.
But what needs to be cited here are the facts that are used, the average number of children and the average generation length. In this case the numbers used seem to be very conservative and unlikely to exaggerate the result, but in general such facts should be sourced. It is far too easy to get the results you want with such calculations by fudging those numbers.
The bigger problem I see with this answer is the extremely simplistic model used, the author mentions the problems with it but provides no estimate on the error these simplifications could have on the result. I suspect that this specific model is too simplistic to be really useful, but I don't really know how big the error in this model is.
Following the math is not the problem, assessing the validity of the model and the facts/constants used in it is the hard part. And that part should be referenced.