"if it is indeed less than a millimetre thick, if it is pushed by the sunlight, then it is maybe a light sail"
It's not clear at all that he think it's likely that it's less than a millimeter thick or that it is a light sail. That's simply "the only thing that came to his mind" to explain the additional force(s) working on it.
> The following October, Avi Loeb, the chair of Harvard’s astronomy department, co-wrote a paper (with a Harvard postdoctoral fellow, Shmuel Bialy) that examined ‘Oumuamua’s “peculiar acceleration” and suggested that the object “may be a fully operational probe sent intentionally to Earth’s vicinity by an alien civilization.”
or another spot where he says
"Every now and then we find an object of artificial origin. And this could be a message in a bottle, and we should be open-minded."
Seems to me like he is saying it could be worth considering that maybe this is an alien object.
> Man that food sure looks tasty.
> You haven't even tasted it! Why are you saying it's tasty?
There are absolutely no claims that it's an alien artifact. What he is doing is providing reasons for why checking out similar solar visitors is something we need to do.
Likely: greater than 50% probability
Possibly: greater than 0% probability
Saying something is likely is a pretty strong claim. You can make meaningful decisions based on such knowledge.