Yeah, basically what has been said above! I do believe even parted out you need to consider rigor mortis. I only started butchering last year, but let my birds sit in the fridge for 2-3 days just in case. I do believe their lifestyle, even young, is a factor, as in if they are running about and getting a lot of exercise, as opposed to housed in a confined space/coop/tractor, the muscle fibers toughen up too?
I think the meat you have packaged is still useable, just put in the crock pot or something. I’ve heard instapots and pressure cookers work wonders!