I think using nutrition to treat Covid, or any disease for that matter, has always been controversial. Based on my understanding so far, in randomized controlled trials, sometimes nutritional treatments work against Covid and sometimes they don't. Vitamin D is a good example of this, as I've written about before (link below if you are interested; title may seem clickbait, btw).

https://medium.com/microbial-instincts/is-vitamin-d-still-a-wonder-drug-for-covid-19-what-new-randomized-trial-and-existing-studies-say-a1237c7a18f5?sk=0c7a03ef63346787d75dbddc9c2dc8da

Because nutritional treatments only work sometimes, not all the time, authorities probably have a hard time accepting them as legitimate treatments for Covid. Put it another way, however, there's no harm in using them and they may even be beneficial in other ways. But I guess not many practices that.