There's no mention of whether the first test (April 18th) might have been a false positive.
That said the distance between his first symptoms and his 1st positive test seem long, which means there could be a false positive. Talking with my wife she said that some in her group have had repeated positive tests for well over a month since first symptoms.
We will start to understand more now, back in March it was very hard (in the UK at least) to get a test if you weren't hospitalised.