The third team took a different approach [1], with results that are both more accurate, less prone to FP, and generally agree with the other findings.

> Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible.

[1] https://www.nature.com/articles/s41586-018-0204-1

After my understanding of the stacking technique as used in other contexts in astrophysics and without going into too much details :

The problem they were facing is that the signal to noise of the images of the filaments was too low to say that they had detected anything in any individual images. However by stacking (adding) images they were able to detect it because the signal grows roughly with N (N being the number of images) and the noise grows with sqrt(N). So by stacking enough images you'll get the signal to noise necessary to say you've detected sth.

  [t]he energy released by a cosmic collision increases as the 
  square of the incoming object's speed, so a comet could pack
  nine times more destructive power than an asteroid of the
  same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html)

With a single observation we can't deduce much of anything concrete except to floor the incidence of these interstellar objects at greater than 0. I'm no astronomer, but I assume models of interstellar objects as they reflect actual risk to Earth wouldn't be very useful without more observations. Whatever the average density in galactic space, I'm betting they're not uniformly distributed. Our solar system is speeding through space that could be littered with clouds of objects.[1] Are we entering a cloud? Leaving a cloud? We can't know without more observations.

[1] There are theories that posit that the ~30- and ~225-million year cycles we see in extinction events are a function of our solar system's orbit in the galaxy, which takes about 200-250 million years. Shorter cycles could relate to the inclination of our orbit (and other stars' orbits) relative to the galactic plane.

Oumuamua interstellar asteroid. 230x35x35m, ~= 280000 m^3

Density assumption: 2 x water. => mass is ~500,000 metric tonnes.

Spotted only after passing the Sun. Assume we'd spot such objects only if they came within the orbit of mercury so are well illuminated. Assume one such object every 10 years (we've not been searching very long with automated telescopes), and we spot all of them.

Mean mercury orbit radius ~ 60,000,000 km

Area of mercury's orbit: 1.1 x 10^16 km^2

Mercury's orbital area x path length in 10 years = volume swept by one visible object in 10 years.

Asteroid velocity ~100,000 km/h

Path length in 10 years = 100,000 x 10 x 24x365. Swept volume ~ 10 x 10^25 km^3

Distance to Alpha Centauri: 4.37 light years = 4.37 x 9.5 x 10^12 km = 4.15 x 10^13km

Sol's "cube of influence" ~= 7 x 10^40 km^3

Cube of influence / swept volume = rough estimate of number of asteroids in cube of influence. Number of asteroids: 7 x 10^14

Mass of asteroids: 3.5 x 10^20 tonnes. Mass of sun: 2 x 10^27 tonnes.

Conclusion: dark interstellar asteroids like Oumuamua are a tiny fraction of the visible mass of the galaxy.

Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?