> If it were deliberately manufactured by a sentient alien civilization, why choose that specific shape? A light sail based probe doesn’t seem to be a sufficient enough explanation for the shape.

Why not? Assuming a civilization is (or was) technologically advanced enough, they would probably be doing lots of things we couldn't understand, especially given how little we know about this thing. He wasn't saying it was definitely, or even probably, a light sail, just that we should entertain the possibility.

Besides, we don't actually know what the shape is, only that its changes in brightness would be consistent with an elongated object, which is the simplest explanation. Or, maybe the object is perfectly spherical but the brightness changes are due to seeing a light sail at different angles, which is extremely unlikely but within the realm of possibility.

Point is the shape is unknown. A cigar shape is one possible explanation that fits the light curve data; a solar sail is another.

Only the solar sail shape could explain how the object accelerated without outgassing

I'd say its unexpected change in velocity is the most interesting part about it.

Antennas for sending data back need a large area parabolic dish. Solar panels also need a large area. Large area seems like a likely feature of any probe.

for a "naturally occurring" process to create something we could suspect as a lightsail, I wonder about something, perhaps a comet, blowing a bubble. That is a layer of something what could form a membrane over something that could off gas. Both triggered by heat. Bubble is created and pops or deflates and the skin blows around for ever.