Any uptick at all is concerning. In fact, a flattening of the decline is concerning.
The implied thesis in the linked article is that Sweden has reached a level of herd immunity due to widespread T-cell responses, and that life in the country continues as normal.
If that is the case, then we would expect the infection curve in Sweden to follow a classic logistic model -- which is more or less symmetric. A rapid rise should be accompanied by a rapid fall.
An asymmetric curve or one that has a "second wave" of any size tends to refute this theory. It would suggest that there was in fact some meaningful intervention that changed: either the initial infection was among a different group (i.e., what the US saw with infections first dominated by New York and then later by other states) or there was an effective policy response that has begun to wane.