We just detected no evidence of outgassing. Maybe it was an inertialess drive, maybe it was a fusion rocket,
or a matter-antimatter engine (although we saw no evidence of those either) but maybe it was just a gas leaking from the body that our sensors weren't tuned to pick up, or sensitive enough to detect.

Now, if it suddenly had accelerated to 20% of the speed of light in a few hours, and exited the solar system in a column of ionized flame, then maybe we would have reason to suspect an artifact, but it didn't. It just altered velocity slightly, just like an outgassing comet might. Who knows what caused it, and we already know a natural mechanism that does exactly that.
There's a lot of things science can't explain, but that doesn't mean that everything we can't explain is a miracle, or an alien action.

Scientists are not lawyers. Scientists believe you are guilty until you are proven innocent. All we can say for sure right now is that there is no convincing evidence to prove, or even suggest, the object was artificial. Now maybe it WAS the mothership, and 2I/Borisov WAS a follow-up mission sent to confirm Oumuamua's finding. But we don't know that. In fact, we don't even know enough to suspect that.