Already wrong, the very article shows that Jena had about 145 verified cases in April 6. And test availability was absolute shit in March/April. It still isn't much better now, but from the relation of cases and deaths in the first and current second wave[0], we can infer that current testing is about 3 times that of April.

Actual cases therefore were at least about 450 in April 6, if we generously assume everyone who is positive gets tested right now. Jena only has a population of 110k BTW.

[0] https://www.worldometers.info/coronavirus/country/germany/