I don’t understand why it would be important to take K2, magnesium or anything else with vitamin D as those things don’t come with sunlight. Maybe the fact of ingesting the vitamin D changes the equation somehow.
His explanation was that if you get your D solely from sunlight you didn’t need to worry about it, because that form of D is actually water soluble. But the D3 supplementation is only fat soluble and had something to do with the difference in the two types. Even though sunlight does produce D3 in the body the two (sunlight vs. supplement) aren’t identical.