Most likely induced by the antibiotic combination. Other doctors substitute another antibiotic for the z-pac for their patients with heart conditions. Article authors probably didn’t investigate or their rent-a-doc failed to inform them.
I think it’s more direct than that, after reading this article from 5 years ago:
https://medsafe.govt.nz/profs/PUArticles/Sep2015/Hydroxychloroquine.htm
But that article doesn’t answer any of those questions I asked. It seems to suggest that the heart issues observed are all associated with prolonged use. And in any case, it suggests that these issues are typically reversible. It doesn’t proscribe the drug for people with pre-existing heart problems, but advises care with its use in that case.