Vitamin C or Zinc helping with covid is just as much pseudoscience as invermiacin helping with covid.

Technically both can help but the situation where they do will almost never apply in developed countries. A severe parasite infection will make it harder for your body to fight off covid so invermectin could help there by getting rid of the parasites. But most people aren't walking around riddled with parasites let alone to the point where it starts impacting their immune system.

Likewise if you are deficient in zinc or vitamin C it will harm your immune system and in that case they could help with covid. But most people in developed countries get plenty of both from their diet. You get vitamin C from far more than citrus, it's also in potatoes, onions, tomatoes, broccoli, peppers, and many non citrus fruits. It's also in a lot of artificial drinks. Likewise zinc it plentiful in meat, dairy, and grains especially in processed grain foods like cereals which tend to be fortified with it. Consuming excess of either vitamin just makes you pee the excess out. If your diet is bad enough that you aren't getting enough zinc or vitamin C then odds are you're going to notice before those two become too deficient anyways because you're going to wind up with something like an iron or magnesium deficiency which are both far more common and pretty noticable.