It's not clear it's acting as an antiviral, in the classic sense of directly interfering with the virus, by directly bonding with and neutralizing viral protein

It's not clear it's acting as an antiviral, in the classic sense of directly interfering with the virus, by directly bonding with and neutralizing viral proteins, or supplying lookalike proteins for the cells to make defective viral particles with. Metformin affects glucose metabolism. It may be similar to how metformin seems to help with HIV infection; an immune cell infected with HIV goes into metabolic overdrive as it starts producing virus. Metformin seems to block that boosting mechanism through its effect on glucose, preferentially on the most over-driven cells. A similar story with certain glucose-fed cancers, which metformin is also used in treating.