Sorry if I was vague. My question was why you would want your meat stock to have a thickener. I mean I don’t premix all my cornstarch with spices and salt since I want to be able to pick the right amount of aromas and taste separately from how thick I make my soup.
It's more of a mouthfeel thing than a thickener. Gelatin, in the amounts found in a good stock, will still be quite liquid when the stock is at serving temperature, but will sort of coat your mouth with flavor even when the broth hasn't been thickened with a roux or cornstarch.
They were pointing out that with meat stocks it depends what went into the pot - a good home made (or restaurant) one typically already has enough to make it “gel” in the fridge. The box types you mostly get in a store didn’t have this, so might want to add if your recipe assumes it.
