This is not just spin. First, Montana is considered a red state in recent political cycles. They have sometimes elected Democrats, especially to the U.S. Senate.
Democrat Jon Tester has been one of their senators since 2007; and Max Baucus began his senate career in 1978 and served until his retirement in 2014. His seat is currently held by a Republican, but Montana has had other distinguished Democrats. Sen. Mike Mansfield, from 1953 to 1977, was Senate Majority Leader for the last 16 years.
So, Montana has had its notable streaks of blue among the red. However, the relevant number in this current analysis is that Donald Trump carried the state easily last November with a 20% margin.
Thus, Greg Gianforte's 6% margin of victory (assault aside) follows what we saw in Kansas and adds to the sense of an important "win" for Democrats. And sets the stage for an actual victory in GA-06 that will send Demoract Jon Ossoff to take Tom Price's congressional seat in Washington. Ossoff was leading in the most recent poll, 51% to 44%.
Journalist Matthew Yglesias, writing for Vox News, said that it "fundamentally represents bad news for the GOP." He quotes from the Cook Political Report to explain that "the GOP underperformed badly in Montana, after a similar underperformance in the special election for Kansas's Fourth Congressional District."
Yglesias continues: "If Republicans are winning in places like Montana by just 7 percentage points, [the final count was actually 6%] then they are in extreme peril of losing their House majority in 2018."
GA-06 is a much more winnable district for Democrats. Trump won it by only 1%, while he won Montana by 20%. So let's have a good win here and set it up to flip the House in 2018.