As I understand it, Sweden did have a response to Covid that involved a massive increase in working at home and with those who felt vulnerable engaging in social distancing. So the cases tended to involve nursing homes and essential workers.

So it seems like they've found up with "herd immunity" relative to the defensive measures that were taken. Which is to say they don't have unconditional herd immunity. If they get Covid slowly down to a very rare rate (and Covid is decline there quite slowly[1]) and normal activity returns, one or another "super-spread events" could reignite the epidemic in a similar fashion to other places where it's been eliminated currently though with the relative immunity perhaps slowing it a little.

[1] https://cv19info.live/sweden/ Look at new cases in log scale. Sweden's cases have declined but extremely slooooowly. 23 deaths a couple of days ago where the high was 172. That a low rate of decline which indicates Covid will be there quite a while.