It's a very unusual shape for an extraterrestrial object to have. They tend towards the spherical, because of gravity. The more elongated something that "small" gets in space, the more likely it is to break apart under the gravitational forces of its own spin and nearby bodies. Oumuamua was remarkably oblong, but not impossibly so. Like a four-leaf clover that also exhibited some weird reflective qualities that could, but do not necessarily mean, that it was made out of metal. It was on the extreme end of the spectrum for both shape and reflectivity, but to bring myself back to earth a bit it wasn't actually impossible in either spectrum.

It's just incredibly unlikely. Space objects, especially space objects that travel, tend toward spheroid shapes.