It's maybe worth pointing out the tumbling is a hypothesis used to explain variable brightness measurements and not something that was observed directly.

https://arxiv.org/pdf/1712.00437.pdf

For all anyone knows the intrinsic brightness was variable.

(This is probably not very likely without some supporting spectroscopic anomalies. But still.)

As for thrusters - that's the same problem as the comet and outgassing hypothesis. No known mechanism can explain the acceleration.

And if you were alien race and wanted to look at a planetary system without attracting attention, something that looks like a tumbling rock with a plausibly improbable trajectory would be ideal.

It could just be a coincidence that the first large proven extra-solar object slingshotted around the Sun in a way that happened to pass close by the Earth.

But considering the size of the Solar System and the convenient phasing of the Earth's orbit, what are the odds? It's certainly very curious.