Flavonoid Xanthine Oxydase Inhibitors vs Hydroxychloroquine
HCQ does show antiviral properties in vitro, but, in vivo (in humans), when taken after an initial dose of 800mg for 4 days at 600 mg/day, it was not found to prevent COVID-19 in high-risk exposure settings.  Thus, it didn't stop people from getting SARS-CoV-2 infection followed by onset of symptoms/severity.
I suspect this is exactly what would happen with flavonoids, many of which have shown antiviral properties in vitro (laboratory settings).  The information on flavonoids and SARS-CoV (2003) is very sparse, but several flavonoids did inhibit SARS-CoV in vitro (see last paragraph below).  I do know there was a clinical trial approved in 2020 by NIH in which the investigators received an IND (From FDA)  to use diosmin as a supplement to treat patients with COVID-19 (i.e. hospitalized).  However, I don't know if was completed.  Diosmin is available OTC as a supplement, and used via prescription under the brand names Dafflon® and Vasculera® for chronic venous insufficiency (CVI), which is caused by venous hypertension (VH). VH, in turn, aggravates metabolic imbalances leading to a self perpetuating cycle of further metabolic changes, including venous acidosis. These changes promote further inflammation in vascular tissue leading to edema, skin damage and possible ulceration and deep vein thrombosis (DVT). Varicose veins and hemorrhoids are also part of the spectrum of CVI disorders.
Interestingly, it took 16 years before a group published the most comprehensive paper on in vitro inhibition if SARS-CoV (2003) by many flavonoids that were considered.  I review these in a pre-print on flavonoids that was just accepted for publication.