Was Walz under activation or active training orders at the time of the event? If not, then he would not be under UCMJ authority. United Sates v Morita; 74 M.J. 116 - there is no UCMJ jurisdiction over a reservist who commits an offense when not in a military status – i.e., on active duty, inactive duty training, or serving with the armed forces.
I have also read that the judge reduced the charge to reckless driving and only gave him a $200 fine plus court costs. So like most things that the pundits and media drag up (left lean or right lean) its a nothing burger.