...I'm surprised it was 2nd degree murder instead of manslaughter as an alternative.
As I read that article, the jury may consider all three: 1st degree, 2nd degree and manslaughter. But per the link they have to find McInernay not guilty of 1st and 2nd degree murder before they weigh the manslaughter charge.
I'm not sure what "before" means. As I said above, I've been a juror in CA where we were given options of 2nd degree, voluntary and involuntary manslaughter (and, of course, acquittal). Deliberations addressed all possible verdicts simultaneously and the straw poll votes changed from day to day. (We deliberated for a full week.)
It's true a jury can't find a defendant guilty of both murder and manslaughter for the same killing, so, yes, a finding of guilty to manslaughter requires a not guilty to murder. But that doesn't necessarily mean jurors start at the top and work their way down.