The paper says that outgassing would not be likely, as if there were outgassing it would apply a net torque on the object, causing a change in the spin rate which was not observed. They suggest that the spin rate would change over the course of a day to several days with some of their built in assumptions.
However, they do make a lot of assumptions: that Oumuamua is spinning on its minor axis, that the dimension ratios are around 6-7:1:1, that Oumuamua hasn’t indeed broken up and is a contact binary already, and also that the observed acceleration (which is tiny, around 5 µm⋅s⁻²) did actually occur which is based only on one month worth of post-perihelion observational data, and so on.
Also, if it were a light sail, they must be incredibly common if we just happened to see one in an automated survey shortly after that survey started up. The galaxy would have had to have been flooded with them.
It does, and that is beyond cool. But it's not the *only* model that can account for the behavior. Outgassing would work, for example. Yet we saw no sign at all of comet-like outgassing from the object. Hence the attempts to find another explanation.
On a purely subjective level, I adore the thought of this object being a piece of debris from some distant star system, the remnant of a shipwreck in space. It's rather like finding scraps of a ship on the beach; you can't help wanting to know the story of where she was from, who was sailing her, and how she was lost.