The big question is about what caused its non-gravitational acceleration? The only thing that makes sense there is a comet - comets are icy and when that ice heats up, such as by passing around the sun, it can result in an outgassing that can create an acceleration. But it showed absolutely no indicators of being a comet, such as the tail or coma that comets display.
Another problem for the comet theory is that the object was very elongated like a submarine and spinning fairly quickly. Outgassing to the point of creating a net acceleration would have further increased this spin and likely torn it to to pieces. For those that'd like to go for the 'its aliens' theories, it accelerated at the exact time you'd accelerate to have the most substantial impact on your orbit. Of course that's also the most likely point for a natural source of acceleration as well.
So ultimately? Nobody knows what it was and while you have more or less plausible hypotheses, all have various flaws. Except aliens, but you can use aliens to explain any and every unknown phenomena. For instance pulsars are rapidly rotating stars that emit a high energy beam. As it rotates this beam, when the pulsar is aligned with earth, can result in it regularly pinging Earth. The first pulsar signal was detected in 1967 and, unsurprisingly, was labeled LGM-1: Little Green Men-1. Regardless, this is one of the many reasons that evolving our space technology should be priority number 1 for humanity. There are a countless numbers of reason that lacking the ability to intercept, or even identity, relatively large objects flying right next to us in space is a very bad thing. The Fermi Paradox being somewhat meaningless in the interim is just one of those reasons.
The level of coma and other factors are related to how close the comet comes to the sun. Oumuamua came extremely close (about 25% the distance from Earth to the Sun), and displayed no such characteristics. This is why thought it was originally classified as a comet, it was quickly reclassified as an asteroid. But then it started accelerating which means it's also probably not an asteroid. And on top of this all a comet would also likely have torn itself to pieces accelerating given the shape and tumble of Oumuamua.
It somewhat cleanly refutes every possible classification leaving only the ¯\_(ツ)_/¯ group of celestial bodies.
Another problem for the comet theory is that the object was very elongated like a submarine and spinning fairly quickly. Outgassing to the point of creating a net acceleration would have further increased this spin and likely torn it to to pieces. For those that'd like to go for the 'its aliens' theories, it accelerated at the exact time you'd accelerate to have the most substantial impact on your orbit. Of course that's also the most likely point for a natural source of acceleration as well.
So ultimately? Nobody knows what it was and while you have more or less plausible hypotheses, all have various flaws. Except aliens, but you can use aliens to explain any and every unknown phenomena. For instance pulsars are rapidly rotating stars that emit a high energy beam. As it rotates this beam, when the pulsar is aligned with earth, can result in it regularly pinging Earth. The first pulsar signal was detected in 1967 and, unsurprisingly, was labeled LGM-1: Little Green Men-1. Regardless, this is one of the many reasons that evolving our space technology should be priority number 1 for humanity. There are a countless numbers of reason that lacking the ability to intercept, or even identity, relatively large objects flying right next to us in space is a very bad thing. The Fermi Paradox being somewhat meaningless in the interim is just one of those reasons.