I loved Rendezvous with Rama, and one of it's central lessons is anything is possible when you talk about alien civilizations. We can't assume to know their motivations and technologies. But looking at the basic physics/engineering involved, if it's an alien vehicle it raises some interesting questions and skepticism. Imagine we wanted to build a similar vehicle.

If we wanted to send people to nearby stars, we'd likely need vehicles as big, or likely larger, than Oumuamua. If we were in a hurry to do it with present day technologies we'd have to build hundreds of SLS boosters to launch payloads so that astronauts could assemble a vehicle weighing tens of thousands of tons in orbit. And then we'd fuel it with tens of thousand of tons of fuel.

So we could build it. What would we use for propulsion? Not chemical rockets, 40KM/sec is only achievable for tiny payloads to fuel ratios with chemical rockets. Maybe we could do it with Nuclear rockets, which were tested and proven to be twice as efficient in the 1960s, ala NERVA.

Even then we'd only be able to use a bit more than half the fuel to accelerate towards our destination, if we wanted to stop there. So 40 km/sec would be extremely difficult to reach given how heavy it would be.

And at 40 km/sec it would take 8,000 years, nearly a hundred generations before reaching the nearest star. Assuming the life support systems could last that long, they didn't run out of food, water, air, etc. That the engines weren't degraded by radiation over 8,000 years so they would still work when reaching the target. It's extremely unlikely we could count on any of this being true.

But if we were ever to launch such a vehicle it would likely be outclassed very quickly. We already know how to build better propulsion systems. Existing ion drives are far more efficient, and theoretically offer over 100KM/sec speeds, we just haven't been able to scale them up yet.

If we can scale up ion drives or similar technologies, we already have designs for Project Orion are entirely feasible, and offer massive speed increases. Freeman Dyson's interstellar designs offer 1,000-10,000 KM/sec speeds. If it took us a hundred years to build one, it would likely pass a hundred year old NERVA ship within a year or two.

My point is we'd never launch such a slow vehicle as Oumuamua, because it would be relatively easy to build a significantly faster one. Even with todays technologies.

So it's reasonable to think that an Alien civilization that could build a ship the size and speed of Oumuamua, could build much larger, faster ships for interstellar travel. If Oumuamua is a ship, the question would be why didn't they?

One possible answer if Oumuamua is artificial, that it's not an interstellar vehicle. It's an interplanetary vehicle that had a complete failure of it's systems, and had just enough velocity to leave it's star system, and has been drifting for tens of thousands of years.

But in that were the case, how did it came so close to our Sun? It's reasonable to assume that many billions or trillions of asteroids and comets have been ejected by star systems, so that occasionally a few come close enough to ours to make the tight orbit around the Sun that Oumuamua did. But it's hard to imagine there are billions or trillions of space wrecks wandering the milky way such that one randomly passes this close to our sun.