That of course is not proof of a causal link, but it's enough to justify follow up research.
What would be interesting would be a comparison again Cialis (the competitor to Viagra).
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.
Or money.
My grandpa whom I've never caught lying, told me that in WW2 Romania, as a very young conscript, he was given a medical examination... consisting of the nurse fondling his junk. Apparently they decided very fast he was healthy and the same for every other conscript, except for the only son of an aristocrat(most were tough peasants such as my grandpa) who would not get an errection.
A couple of weeks later, that young man who did not pass the "physical exam" got really ill and only my grandpa took pitty enough to carry him on his back to a hospital, where they guy died a couple of days later.