An interstellar asteroid ? I'm far from convinced

Using computer simulations, Connors and his colleagues found in 2017 that Bee-Zed's orbit has been stable over the past million years. The discovery took Namouni and Morais by surprise; their previous work had suggested that orbits like BZ509's could only last 10,000 years or so. To push these results further, Namouni and Morais built a model of our solar system in its current layout. They then sprinkled in a million virtual “clones” of Bee-Zed, each with a slightly tweaked version of the asteroid's observed orbit, and ran the simulation for the virtual equivalent of 4.5 billion years.

Many of the clones eventually collided with the sun or were ejected from the solar system. Half of them lasted less than seven million years. But 46 of the clones were stable over the lifetime of the solar system—and 27 of them closely resemble Bee-Zed's current loop.

For humans to have a statistical chance of seeing Bee-Zed, Namouni and Morais argue that the asteroid must have been in a highly stable orbit for 4.5 billion years. But if Bee-Zed has orbited the sun since the solar system's childhood, how did it end up going backward? After considering and rejecting a variety of potential explanations, they say it must be an interstellar interloper.

I may or may not actually the paper. Until then, based on the press release, this is daft. All their results say is that asteroids on this sort of orbit can be stable. That's all. I don't see any reason whatsoever that the asteroid in question must always have been on that orbit since the Solar System's formation : it's possible, but that's no reason to think that it actually happened. Maybe it's been in this orbit for a long time, but why assume the full length of the stability ? Why can't it have been there for say, 500 Myr ? More importantly, to estimate the chance of a detection you'd need to know the rate at which asteroids are likely to be scattered into similar orbits. A low probability of an orbital capture could be countered by a high number of asteroids.

Then there's the preferred explanation of the object being interstellar. Is it more likely that such an asteroid should enter this orbit than one from our own Solar System ? I'm not seeing any obvious reason why this should be the case. We need a probability comparison at the very least. And since the flux of interstellar asteroids is only known to be greater than zero, I don't see how that's currently possible.

https://news.nationalgeographic.com/2018/05/interstellar-asteroid-jupiter-bz509-astronomy-space-science/