Here is the deal as I understand it. There are a few factors at work here. Yes the bird needs to have time for the actin and myosin to break down so it is not stringy in texture. Over the years we have come to not believe in our teeth so much it seems, if you have to actually chew on something now it is said to be tough, back then as long as it could be chewed it was not tough. Also with the food safety we are more aware of now meat is cooled quickly and that prolongs the time it takes for the breakdown to occur. This is mostly due to the large numbers or critters we do at a time, and that machines and equipment are used that can cross contaminate. I often hear stories of the chicken for that nights dinner hanging on the back porch for the day, often not plucked or gutted till evening. This will greatly accelerate the breakdown of the meat and is still done in many cultures, that bird will cut with the edge of a fork after cooking even if it only hung for an hour. An ice chilled or even water chilled bird that was gutted right away will hold the muscle structure for a day easy. Then there is the cooking too, anything that seals the bird so that the moisture in it breaks down the muscle in cooking will make it tender too. Sealing it in grease would do that for sure, covered cooking like a roasting pan with the lid on for the first half of a long slow cooking would too, those were common ways to cook then. Now we demand a skinless bird part that can hit the grill of the BBQ hot and dry and still fall apart easy when we are done cooking it in only 10 minutes.