I had the same problem and have worked it out.
Look closely at the countersink bit in the photo - it's a countersink 'fit over' with a grub screw, which affixes to a regular drill bit.
They're just using the 'fit over' piece and drill bit facing towards the drill, and using the drill in reverse presumably.
I.e. it's this: https://smile.amazon.co.uk/dp/B0001P120G/ref=cm_sw_r_cp_apa_glt_fabc_95ACYKPBNTQ8EEA0BMMH?_encoding=UTF8&psc=1
Not this: https://smile.amazon.co.uk/dp/B00AHGREDI/ref=cm_sw_r_cp_apa_glt_fabc_Q16CFY780WRKZDV74C4A