So in other words, the probability is not Zero. Meaning it is very much worth considering and this article is legit.
That same probability applies to the light sail.
On the other hand, we have essentially no knowledge about what can possibly be flying around out there and thus your claim that it is extremely unlikely to be an artificial object is not based on anything but your own conviction that this is the case.
Probability allows you to say something with confidence if you have information to work out the probability. If you have no information then you have no probability.
I think that our knowledge of the universe does allow me to conclude with some degree of certitude that it is highly unlikely to find a whale on an extra-solar trajectory.
On the other hand, we know nothing about the sort of objects that could exist out there and about the possibility of alien civilisations.
I am not saying that it is likely or unlikely to be a light sail because the fact is that we don't know enough to assign a likelihood. That's as unbiased as can be.