So the first part of this is the direct answer to what you asked, and the second part is about "the why", about what's going on "under the hood" in these recipes to make them work.
You can use cauilflower (in that recipe, any oil will do instead of coconut oil), tofu, pumpkin seeds or hemp seeds (or any other seed, if you aren't allergic to them), and even simply any plant milk on its own, reduced into a sauce.
You can also use potatoes as the base for the sauce, but I think those are less successful (they usually taste thinner or starchier, instead of creamier).
anything I can substitute... to get a similar end result.
Nuts and coconut are high in fat, which is why they make such creamy sauces when liquified. In general, you can imitate that texture using other ingredients in 2 ways:
1) Use added fat. You can replace the fat content of nuts or coconut with another source, like vegetable oil or vegan butter.
2) Use a thickener. There are all sorts of thickeners with different texture properties: flour (recommended), potato starch (recommended), cornstarch, arrowroot powder, tapioca starch (not recommended for Alfredo sauce, it produces a stretchy, cheese-like consistency that's good for cheese sauces but not cream sauces), etc.
As far as imitating creamy taste of dairy cream or of cashew cream, that's little bit hard, because there aren't many things that taste as close to dairy cream as blended cashew will. But you can still produce delicious things, so just experiment. Sometimes adding a tiny amount of a sour ingredient (like apple cider vinegar or lemon juice) or a sweet ingredient (like sugar) will help change the flavor to what you want (dairy often has both sour and sweet notes, because lactic acid is sour, and lactose is slightly sweet). If you don't know how these things will change the flavor of the sauce when you're cooking it and are afraid of ruining it, you can take out a small portion of the sauce from the pot, and experiment with seasoning it (that way if it goes wrong, you haven't ruined the whole pot).