If you plot NY & NJ versus the European countries, it will look very similar. If there is not a second wave in the hardest hit areas then it will suggest that something like this might be at play and herd immunity has been reached (20% cumulative infection rate + 50% of people naturally immune due to T-cells = 70% not susceptible to spreading the virus).
In summary, the article is saying that people who've had a common cold coronavirus recently enough that their immune system remembers it might have resistance to Covid.
This reminds me of something I was reading about a month ago. With some similar viruses exposure to one virus in a family provides some resistance to other viruses in the same family. In layman's terms, the immune system recognizes the new virus as being similar to the virus it fought off in the past and it starts fighting back sooner than if it had never seen the first virus. A great example of this is small pox and cow pox. Before small pox vaccines became a thing, farmers would bring cows with cow pox into town so people could touch the cow with their elbow(keep reading, it'll make sense), and catch cow pox. Cow pox was pretty mild to a human, left a rash/scar on the site of infection (that's why they chose the elbow), but if that person ever got small pox they were likely to have a very mild case. The immune system recognized small pox as being similar to cow pox and jumped into action before small pox could do serious damage.
It is possible that recent exposure to the very common & less dangerous coronaviruses would provide resistance to Covid19. It would also help explain why ~50% of people infected with Covid19 have no symptoms.
The million-lives question is whether those 50% can still get other people infected. If they can then that isn't true herd immunity. It's actually the complete opposite, instead of slowing down the virus (herd immunity), they would be accelerating the virus because an infectious person living a normal life will likely come into contact with more people than someone who is at home sick.
According to various studies, asymptomatic carriers of Covid could be responsible for 10-40% of the spread of the disease. My takeaway from that is that their immune systems might be catching it fast enough to prevent them from getting really sick, but they could still be passing it to vulnerable populations.