Some antifreeze products are sold "premixed" with water, so they are the equivalent of mixing equal parts parts of full strength antifreeze and water. Antifreeze is antifreeze, water is water, and a 50/50 mixture of the two is called "coolant" since a 50/50 mix of full strength antifreeze and water is what most OEs call "coolant".

These products are "convenient" because they require no mixing, but they are rather expensive and it is not that hard to add equal amounts of full strength antifreeze and distilled water when doing a coolant change. Unless there is a leak of a faulty pressure cap, it is unusual for the coolant level to drop between three year changes, unless the car is driven more than 30K miles.

If you start with a 50/50 mix and at some point the reservoir is a bit low and needs a few ounces, using either distilled water or straight antifreeze will not have a major impact on the desired 50/50 mix, which is the optimum tradeoff between corrosion inhibition and thermal capacity.

Duke