I don't know about magnesium specifically but I know there were experiments with calcium showing that supplementation both a) didn't absorb 100% of whatever you took, and b) had an upper limit for what could be absorbed over N hours. At a certain point you just have to eat food with calcium in it. I wouldn't be remotely surprised to find that's true for most/all micronutrients.
Supplementation can help, sometimes, but will always be a fraction as impactful as a healthy, well-rounded diet.
Calcium and magnesium share some pathways for absorption and compete with each other. Not only is there a limit to how much calcium or magnesium you can absorb, but if you have a high-calcium diet, your body will absorb less magnesium.
There are other minerals which have this kind of competition, so supplementing with one mineral can cause problems with absorption of another.
IIRC, calcium uptake from the intestines is boosted by the presence of fatty acids. Different types of magnesium supplements have different absorption rates (e.g., magnesium oxide vs magnesium glycinate).
Even to the extent that that may be true, it is going to be an average: different people's bodies will be different in what they can absorb, and the same body at different times will be different. Sometimes dramatically, particularly if you have a deficiency.
If you do a lot of sport, you should take more than that?