If one googles the term "ballast flight" one finds lots of studies and professional papers documenting that high speed trains exert dynamic forces on the trackbed. That's pretty understandable considering the basic rules of aerodynamics (which I don't pretend to understand) and the reality that the underside of a high speed train can only be streamlined so much thanks to needing to have bogies and wheels sticking out downwards,
A major concern is not so much flying stones hitting things as general loosening of the roadbed and deterioration of the ballast..
It's pretty obvious that there is no "right" or "wrong" here. Some operators have found other design strategies while some rely on other types of roadbed, some are content to stick with rock ballast, or some altered form of such.
Which solution Alto chooses is yet to be seen, but we can be sure that their engineers will consider this issue and make decisions.
We may all have our opinions, but there is lots of middle ground here. Arguing "is" vs "is not" is a bit pedantic,
- Paul