600 IU of vitamin D isn’t going to do anything except keep you from getting rickets.
If you have a vitamin D deficiency, it will not reverse it.
The people getting 100,000 IU had blood tests so they knew they would not be overdosing them (no one should take that amount without medical guidance, also this is a one time dose and not an ongoing supplement.)
To give you an idea, the reccomended amount of vitamin D for infants is 400-1,000IU. (Depending on who you ask.)
If you think you have a vitamin D deficiency, get tested and the doc will tell you how much to take. 600 IU is not going to even budge your vitamin D levels.