It's what I heard on a corona virus podcast with a virologist from my country. The standard test used for the corona virus also detects dead virus. It is the suspected cause of the media reports form a couple of months ago which claimed that recovered covid-19 patients had contracted the virus again.

The question of when it is safe to release a Covid patient was a big matter of debate when the duration of quarantine measures was being discussed in late March and April. The situation was quite critical at the time and the standard was 14 days back then.

However the virologist discovered that two weeks was likely not necessary because tests had shown that the patients with mild cases were no longer contagious after day 7 of the disease. But it is difficult to make a political decision to shorten the quarantine period if the corona virus test itself is still positive at that point. So I think they kept it at 2 weeks despite doubts whether or not that was really necessary just to be safe.