Those tests, and all the tests we are seeing results for are RT-PCR, not serological; i.e. they are only detecting those who have active virus in their system, not those who have been exposed and successfully fought it off. The article is dead on, this is what we should be doing right now to figure out the appropriate response. I also suspect the mystery, non-flu illness going around the Northeast since late January was in fact Covid.
Either it spread almost instantly and saturated whole populations, or else it is spreading at a finite and measurable rate. You cannot have it both ways.