Because his attorney's narrative is being given after the case has been concluded, and judgement rendered.
While the case was still open, and the plea deal was still offered, the court did not accept that narrative. What the court accepted was... left in limbo... depending on what SSgt Wuterich "chose" to plead.
The law pretends not only to power - but to reality itself. If SSgt Wuterich chose to stick to the literal truth, the court would choose to believe literal lies, and then through it's power, declare the truth to be lies, and it's lies to be truth, and use that ruling to utterly, ocmpletely and absolutely destroy his life.
On the other hand, if SSgt Wuterich "chose" to accept a "small lie" as the truth, then the court would also accept that "small lie" as the truth, and rule it to be the truth, and then, through it's supreme generosity, not use it's absolute power to utterly destroy his life.
And this is the endgame, remember, after threatening to absolutely destroy his life, and infamously slandering him, for six years.
So now that that's concluded, his attorney can assert, against the ruling of the court (but with a certain amount of political risk) that he told a "small lie" to save himself, but that it was still a lie because he was completely innocent.
THIS is what passes for justice.
It is now our duty, as citizens, to not only remember how twisted our system of justice has become, but to act on the knowledge.