The sentence sounds like there isn't particularly much evidence against him... (or the standards in US military courts have changed significantly).
1. It was a court martial. Standards required for conviction by a court martial board are significantly different than in a civilian court.
2. The sentence was neither harsh nor light. If someone had been injured or the aircraft damaged then the consequences would have been tougher. The BCD means no veterans benefits...and a very difficult time finding a job.
3. There is also an administrative action occurring, a financial liability investigation for property loss (FLIPL) investigation. He is likely liable for tens of thousands of dollars for the damage/destruction of the vehicles as well as the air items. This is purely an administrative action and will follow him into civilian life. His tax returns will be dunned until the debt is paid.
We don't throw folks in military correctional facilities anymore for things below a capital crime. Not cost effective.