But have they even asked? I have no doubt that once Prof. Loeb's team have finished their extensive analysis and published the samples will go through an extensive peer review process as is the norm. This process could and probably will take a couple of years.
But posting online conjecture irrefutably claiming the entire process is without merit without having done any counter-research does nobody any favours. Its akin to claiming the rest of the planet doesn't exist because you have never been or seen beyond the horizon.
Now it may be that some of the samples collected are of terrestrial origin. It may be that the research doesn't refutably prove anything either way. But as scientists we should wait until all the hard work has been done before making our minds up either way.