Boiling chicken?
I think the reason the recipe wants the chicken cooked this way is so that the texture will be “soft” and shreddable and the flavor will be chicken and nothing else. That will probably let your sauce and/or cheese or whatever stand out. I actually prefer to steam chicken for this sort of thing. If you have a metal vegetable steamer, put it in your saucepan with water below and the chicken in the steamer basket. Cover it and cook until the chicken is cooked (about 15-20 minutes) You may need to add more water because of the long cooking time. And, yes, you can start with frozen if you cook them this way.
