Thing thing about statistics is that all answers really really really depend on your assumptions. And "statistical answers" are meaningless unless you carefully lay out assumptions and those assumptions are correct.
That being said, this seems to be a fairly straightforward question as long as you don't care about having a meaningful answer. Let's assume:
the comet was either made by nature or a hypothetical alien civilization
the alien civilization creates rock shaped objects at about the same rate as humans
the average human produces one rock shaped object in their lifetime
the alien civilization has existed since the beginning of the universe
the ratio of tools shaped like this to all tools is the same as the ratio of natural comets shaped like this to all comets
the comet is roughly a sphere 1 mile in diameter
the comet was randomly chosen from a population of similarly sized objects
the comet has the same density as Halley's Comet
the ratio of comet mass to star mass in our solar system is the same of that of the universe
We can be pretty much certain that most these assumptions are wrong, but let's move forward. The expected number of alien-made asteroids is 1.5 x 1018. The expected number of natural asteroids is 3.1 x 1038.
Both these would need to be multiplied by the ratio of objects shaped like this to all objects, but that's a moot point since all we care about is the ratio.
So, approximate odds that it's not natural? I'd say 1.5 x 1018 divided by 3.1 x 1038, which is 4.8 x 10-21
That's 0.00000000000000000000484.