In this case, I don't really know much about this either way, but 30 seconds of effort by just copy/pasting the exact sentence you quoted into Google found me something that seems to fit the bill, showing that contaminants in sequencing is "pervasive" and must be accounted for--such as by filtering things that look out of scope--to get reasonable results.
https://bmcbiol.biomedcentral.com/articles/10.1186/s12915-02...
> Contaminant DNA in bacterial sequencing experiments is a major source of false genetic variability
> We found that contamination is pervasive and can introduce large biases in variant analysis. We showed that these biases can result in hundreds of false positive and negative SNPs, even for samples with slight contamination. Studies investigating complex biological traits from sequencing data can be completely biased if contamination is neglected during the bioinformatic analysis, and we demonstrate that removing contaminant reads with a taxonomic classifier permits more accurate variant calling.
Can you provide a source for this? There are a lot of cranks right now claiming that there's no such thing as a virus because we've never been able to get a non-contaminated sequence (something I've assumed was due to culturing). If this is true of everything there's no limit to what can and cannot exist per their insane arguments.