It did cross my mind that the sample in the test mentioned above wasn't huge. According to some research I found, 5.69 deaths per 100,000 of the population in England died from MND between 1998-2019. That is skewed slightly towards men. If Doddie Weir was the only MND death amongst the rugby players in the sample, that, I think, would make the sample appear massively worse amongst rugby players.
I'm no statistician but in my view that would need a much larger sample.
I think the link to dementia is much less tenuous though.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9344929/ refers.