"When the bug flares up again"?
You're missing their argument.
They're claiming that they have herd immunity. If true, then the bug will not flare up again.
There is speculation that, because the disease was allowed to spread more freely among those least vulnerable, Swedes may already benefit from a degree of herd immunity.
You can drive a truck through the weasel words on that sentence. And it's easy to see why...because there isn't any hard evidence to back up his claim.
If you click through to the articles, you'll find The Blaze's reporter breathlessly claim "Sweden's chief epidemiologist now believes his country has achieved herd immunity."
But....not quite...clicking further to the underlying Bloomberg article - where you DO get a quote from Anders Tegnell, Swedend's chief epidemiologist (who I trust a lot more than Fauci and who I think did a decent job of running the show, even if I fundamentally disagree with putting all my eggs into one basket) says "On Tuesday, Tegnell said Sweden “probably” has achieved a fairly high rate of immunity, which he predicts will protect his country from new outbreaks."
This encapsulates the problem in tracking Sweden and Coronavirus - bad reporting and people with agendas. There are lots of people saying that they THINK that has happened - some of whom aren't in Sweden - but when you get an accurate report from Dr. Tegnell or someone in authority, you get nothing like what the talking heads are saying.
It is entirely possible that such a thing has occurred in Sweden, but I doubt it since it's population density isn't anything like that in NYC. I saw a research studying suggesting herd immunity may have been reached in NYC but you have to slog through a lot of models and analysis and theorizing.