As several people here have already pointed out, and as was mentioned in the article, the permafrost is already melting. This research is being conducted BECAUSE people are concerned about viruses thawing out and becoming infectious again. They're looking to see where the viruses are, what kinds we'd be dealing with, and whether they even would still be viable after so long (as this test determined, the answer is at least sometimes yes, which is good to know!). And the article also specifies it's a virus that only infects amoebas - they specifically chose it because they didn't want to risk reviving anything that could spread to people or animals.

Everyone's so eager to believe scientists don't know what they're doing, I stg. I blame Michael Crichton.