Well, a year later we know how that worked out!  There is no long lasting immunity. If Tegnell had 'googled' coronavirus, he may have found the results of a 7 year study done in Kenya about whether immunity was ever developed against another coronavirus that causes cold-symptoms, which resulted in the answer: NO.  Almost all participants contracted the virus at least twice, one man caught it 8 times, and the shortest amount of time between infections was 3 months.
Herd immunity can only occur with viruses in which permanent immunity is developed, naturally by transmission (chicken pox/measles) or created by a real vaccine.  For measles at least 95% of the population must be vaccinated for that to happen and in Sweden they were arguing that if 40% of the population "developed immunity" that would be enough to 'get rid of it'.
The results have been catastrophic!
Further, if Tegnell had bothered to read anything about Sars-Cov-1, he would have understood that MANY people who were hospitalized in 2003 epidemic were left with permanent lung damage and chronic, debilitating illness.  Just like now.