It's a slow cooker recipe.
Interesting! I've never used milk in split pea soup. Adding it ahead of schedule certainly won't do any harm, but you may see some "separating" as the milk proteins cook. My guess is that the natural creaminess of well-cooked split peas will mask most separation, and a good stir can handle the rest.
I have been eating and making pea soup all my life, and have never heard of adding milk in the recipe. I would have omitted it altogether.
not to mention I cannot imagine cooking it for 5-6 hours!