It doesn't really say that: it just repeats the standard advice that 4000 IU is the safe upper limit. The real answer is to get yourself tested regularly if you supplement large amounts of D3. (I'd advise taking K2 with it too, but do your own research.)
My question is, does the k2 supplement make a difference? I'm asking this as someone who eats regularly a lot of greens.
No, they said to avoid taking more than that unless your doctor recommends it. No mention of safety.