It means people with ALS don't get Alzheimer's according to a correlation. Now, how is the inherent lifetime and lifestyle difference bias corrected? No idea.
I think it's actually a poorly worded sentence and it's saying that there's an anticorrelation between gene length and the relative expression levels between healthy people and those with ALS (and also between healthy people and those with Alzheimer's). This means that ALS and Alzheimer's both have similar effects as aging does and decrease expression of longer genes, according to their study. The sentence before it seems to be specifying the "anticorrelation" that they're talking about and the sentence in question is saying it holds in ALS and Alzheimer's as well as in aging.