It didn't take long to link Crataegus to lisinopril:
2015 Nigeria / Hibiscus-Lisinopril Comparative Study
HS reduced serum ACE and PA in mild to moderate hypertensive Nigerians with equal efficacy as lisinopril. These actions are possibly due to the presence of anthocyanins in the extract.
pubmed.ncbi.nlm.nih.gov
'....These actions are possibly due to the presence of anthocyanins in the extract.'
They're right. The main active constituents of Crataegus for heart rhythm maintenance are anthocyanin/anthocyanidin.