Did you also practice strict biosecurity during that time? Infected Marek's dander dust can be carried on clothes, shoes, hair and skin. If you tended your other birds and then tended the chicks you could easily have introduced the Marek's virus to the chicks before the vaccine had time to take effect. Silkies are very prone to Marek's so it is highly likely that is what your rescue bird had. The disease has these long spells between outbreaks. It can be several months or even an annual event as chicks reach the vulnerable age.
I hope the director of your Texas lab is correct but the circumstances are pointing more to Marek's with that information you just provided about the rescue silkie.