As I read the story, the texting-related charge was only for injuring the girlfriend. The motor vehicle homicide charge was unrelated to texting.
The article wasn't clear, but for all we know, most of the sentence was for killing the guy, and the texting charge got him an extra few hours of community service after he's released from his year in jail. (The only texting-related MA traffic law I could find carries a maximum penalty of $100 for a first offense, and no jail time. The motor vehicle homicide law, on the other hand, provides for the maximum 2.5 years in jail and the 15 year license suspension the guy got.)
And I don't see any reasonable doubt on the vehicular homicide charge. His own testimony is that he was distracted, and following a car so closely that when it suddenly braked, he turned into oncoming traffic. He admits to multiple instances of incompetent driving right there, and it led to a death. Even assuming he's being honest about the texting, his own "defense" merits the punishment he got.
So my guess is that despite the article's emphasis, the texting law got the guy a wrist-slap, and his actual punishment had essentially nothing to do with it.