Some quotes from that article:
The driver who caused this harm is under the age of majority per Texas’ penal code. Juveniles are excluded from many elements of accountability that are available within Texas’ criminal justice system for adults.
In Texas, criminally charging someone under the age of 17 as an adult requires a process culminating in certification by a County Court at Law judge, not simply the willingness of a prosecutor to file such charges at their sole discretion.
The Waller County D.A. and Special Prosecutor are neither a judge nor a jury. However, they did take their criminal investigation to an empaneled Grand Jury last week....
....a Grand Jury who would then decide whether or not to indict the Arnolds’ son for the destruction he caused while behind the wheel of their diesel truck that day.
Evidently the Grand Jury decided in favor of the indictment.
And there's this question that the lawyers for the 6 injured bicyclists will be pursuing:
Who physically modified the engine of the Arnolds’ diesel truck which they permitted their son to operate to assault cyclists and crash into our clients? (We understand that in order to “roll coal,” engine modifications must first occur in one of two ways: 1) Increase the fuel flow of the mechanical diesel pump. This way, when the operator presses the gas pedal, the smoke billows from the exhaust pipes, usually for a limited amount of time, or 2) By restricting the airflow to the engine’s intake. The toxic, blinding, smoke is the result of incomplete combustion, therefore removing oxygen will cause the desired effect.)