So, are you using rotisserie chicken from the grocery store? Or am I wrong to assume the shredded chicken is precooked?

Yes - it is cooked, shredded chicken. thanks!

The chicken in the recipe must be precooked. You are only cooking it a minute or two to reheat it, not enough time for actual cooking of raw meat.

I would think your assumption that it's pre-cooked is correct, since 1) it is easily shredded, and 2) because it is added at the end and cooked only briefly. Leftover or rotisserie chicken seems about right.