I think a mostly vegan diet would probably be the best you can do as an individual to protect yourself from severe covid (aside from vaccination).
I know two vegans, who don't know each other as it happens, and both of them have what I'd call a shitty diet.
Sure, everything they eat is vegan but have you seen the contents of some vegan products? There are many things packed full of "filler". Especially stuff that comes in a box (although that's true of regular processed food too)
It's not like vegans just eat a steaming pile of broccoli each meal.
I'd say a diet that makes you healthier would be more appropriate: for some, that would be a high-animal-fat diet, others might be something else and so on.
Edit: I get your point: eating healthier is key but to say vegan is probably the wrong way of putting it.
It depends on you know. Until recently (the last couple of years), vegans did tend to have very healthy whole-food diets, for the simple reason that processed foods were rarely vegan. That is different now, but I can see why people associate such a diet with veganism.