// guilty of an “apparent failure” to properly apply the law when he failed to give a military jury the option of convicting Alexander Blackman of the lesser charge of manslaughter //
There was no jury, there was a Court Martial Board consisting of 7 members. They found him Guilty of Murder which carries a mandatory life sentence. The job of the judge is then to determine the minimum sentence that he would apply, which was 10 years, not life.
The Judge apparently should have given the Board the option of guilty of manslaughter, for which the maximum sentence is Life. Even if that had happened, the Judge is likely not to have sentence him to life, but to something less. As the Judge had determined the punishment be 10 years, then he would probably applied the same tariff to a manslaughter charge.
So far from being a miscarriage, it is a legal technicality.