They are quite definitely talking about colonisation of the impact site by deep-living terrestrial microbes - about 300 million years after the impact.
So, no. There is no suggestion that the microbes came from anywhere but deep in the Earth, and they even suggest that their methods can be used to avoid the mistake of thinking that the microbes arrived with the meteor.
The permafrost is definitely melting. It will definitely contain. microbes. It could contain microbes capable of infecting humans. But I don’t think anyone died from infections caught from the frozen mammoths that have been dug out of there.
I can’t rule out the possibility of there being something nasty out there, but it doesn’t seem a huge risk.