They claim half the population is already infected. So conduct a random sample, test 50 people and you would expect 25ish MTO be positive, with most showing no symptoms. If so, then herd immunity is indeed a thing.

I have my doubts that the numbers would support this claim. And if so, then virtually everyone in Spain or Italy would already be a carrier.

The fact that cases were linked to known arrivals also is evidence against this hypothesis : if a high proportion of carriers were unwitting and asymptomatic you would expect many of those diagnosed to not have a link to someone previously diagnosed.