Here’s a possible explanation for why the breeder doesn’t allow visitirs and what they mean:
If a person had birds at home, and one of their birds was sick, that person could bring on their clothes, shoes, or body some pathogen(disease causing organism or virus). Maybe that breeder had an unexplained outbreak and believes it was linked to a visitor to their site who brought in the pathogen.
I have not heard of birds getting covid, but only mammals. But perhaps I am wrong. I have been watching reports of non human animals who become infected with covid and have only seen mammals infected.
I used to study a bird virus and would change clothes entirely and leave shoes outside before coming home. We also showered in and out after working with live birds. If I was a breeder I would make visitors wash up and clean their shoes or wear booties. I’d ask them to wear clean clothes or maybe supply a coverup.