While HCQ is definitely not for us afibbers and possibly not for COVID-19 sufferers it seems that they used high to toxic dosing if the British prescribing guidelines are correct (British National Formulary)?
I looked into this before briefly, as with other drugs effecting the Heart, the risk of QT prolongation goes up with the dosage. At the normal dosage of 200mg X2/day, the risk is relatively low.