Mg is available from the malt, but I have a paper that researched yeast performance and they confirmed that yeast flocculation performance is significantly enhanced with at least 5 ppm Mg in the wort. It would seem to me that this researcher's results would have already had Mg in it if they used a malt wort, but I don't remember if it did.
This doesn't provide definitive guidance on Mg's effect of yeast growth and performance, but I believe that it doesn't hurt as long as the Mg concentration is kept moderate. You will start to have taste impact at concentrations of 30 ppm. Therefore, I recommend that a preferred Mg range is 5 to 30 ppm in the mash and sparging water.
I am still impressed with Dave Miller's Homebrewing Guide, even though its over 15 years old. The Continental Pilsener book is another situation and aimed at a particular style. In the Homebrewing Guide, he does say that 10 to 20 ppm is desirable and the max is 30 ppm. I agree that in that style, it has delicate flavors that would probably be affected by the harshness that Mg might apply. I agree with the recommendation for that style, but not in general.