Thank you for pointing that out. I just checked and apparently, the study authors stated that "Endogenous reverse transcriptase (RT) activity has been observed in human cells, and the

products of reverse transcription have been shown to become integrated into the genome," in line 46-47 of the preprint.

But the study inserted HIV-1 RT into the human cells and overexpress it, so the results observed may not reflect normal human cells.

And, from what I understand, the novel coronavirus does not have reverse transcriptase.