Is it true that under certain conditions, Mg can reduce SiO2?
Yes it is true.  

Conditions are temperature in the 650 to 850 degrees C range.  

There is a cool looking youtube video of the reaction, not that that makes it true.

For a more serious discussion see Production and Purification of Silicon by Magnesiothermic Reduction of Silica Fume

and Ordered Mesoporous Silicon through Magnesium Reduction of Polymer Templated Silica Thin Films