Consider the statistics of it being an alien probe. Now consider the statistics of it being a rock. It's a rock.
We don't know it's a rock. We don't really know what it is, other than it's the first thing we've picked up from an interstellar trajectory, is bizarrely shaped, didn't reflect the pull from the sun, swung by the Earth and then accelerated away.
If you believe it is a probe you must assume an alien civilisation.
Occam's Razor suggests fewest assumptions win.
Supposedly it accelerated a bit, inconsistent with what we know about how rocks move through space.
No decision needs to be made. We don’t know if it’s a rock or anything else and that’s ok.
The basic weirdness is this: it didn't move like a rock, and anything normal that outgassed from it would have been visible, but it wasn't.
There have been a couple hypotheses for weird things it could have been made of, but those ideas look unlikely for their own reasons. One for example was that it was frozen hydrogen, but that would have evaporated in interstellar space. The earth has not shattered because most people assume there's a reasonable explanation that nobody's figured out yet.
We also now know that planetary systems are incredibly common, it is almost certain that Oort clouds are incredibly common. And while stars rarely collide, they do commonly come close enough to gravitationally interaction, and it is likely that material would be flung into interstellar space by that. Our imagination of interstellar space being just gas and dust and otherwise empty is likely very incorrect.
The wikipedia article for that is overly precise and lists 24,200,000 km (0.16176700836 AU) and 15,040,000 mi (0.16179731467).
As others have argued in this thread, once you do this kind of analysis there's nothing left that is odd and needs explaining.
I don't think we'll ever truly know enough about it. Even as just a rock, there's a ton to learn there! And sadly we'll only ever have a small slice of data about it.
Of all the interstellar objects like this one we have observed, how many were rocks?
The fact is, if you consider the statistics of it being a rock that also behaves in the peculiar flight pattern that this thing took, it could very well be a probe.