We don't know any such things. Viagra has effects that could provide a plausible mechanism for a direct causal link. The article in particular points out that it directly interacts with amyloid and tau proteins, that are frequently implicated in Alzheimer's, and that they've explored it's effect on tau in vitro.
That of course is not proof of a causal link, but it's enough to justify follow up research.
Hence why it requires further research before we can claim causality. My point was simply that assuming we "know" the causal link is something else is also unjustified without further research.
Especially given the size of the effect combined with the known interactions between Viagra and Tau proteins.
A comparison with Cialis might well be worth it down the line, but I don't see what it'd give before they've established whether or not there's a causal link.
That of course is not proof of a causal link, but it's enough to justify follow up research.