This is an enormous misunderstanding of what herd immunity actually is. It isn't just fewer people to infect, slowing the infection rate; that happens to some degree with almost every virus in history, and yet very few can be said to reach herd immunity. It's a level of immunity which provides society at large significant protection from the likelihood of outbreak.The indication that the antibody numbers they've reached are meaningful toward herd immunity IS that it comes alongside similar drops across Europe. All of western Europe and the US northeast are experiencing the effects of herd immunity. That is why there's a bell curve at all. If you look at the graph of new cases, the graph in the flatten the curve image, the slope of the graph is an illustration of of the changing R value of the virus. When it's going up, R >1, when it's flat R=1, when it's going down R<1. R depends on a combination of factors: the natural qualities of the virus, the behavior of the population, the immunity of the population, etc. If your measures to stop the virus are sufficient to drive R below 1, you'd still see a short term increase because of the lag from the incubation period, but after a week or so the number of new cases would immediately start dropping. Look at South Korea for that, they started responding near the end of February and cases dropped precipitously about 1 incubation period later. If your measures are insufficient, you continue increasing. Europe did that. European countries had policies that slowed the rate of spread but did not push the rate of growth below 1, the number of cases continued to rise for a month or more after measures were put in place. So why did the number of new cases come back down? Either the people in every hotspot all decided to behave more responsibly over time to slowly drive down the rate of growth in exactly the pattern you'd expect to be caused by herd immunity, or the obvious explanation that there were fewer people available to infect.
If people's behavior is constant, you expect a bell curve as people become increasingly immune. Better control measures don't give you a smaller peak of the same duration, it either gives you a smaller peak with a much shorter duration because you've cut the virus off (but your population remains susceptible to outbreaks indefinitely sans vaccine), or it gives you a shorter peak with a longer duration that the total number of infections is the same but spread out. Nowhere in Western Europe is the pandemic behaving as though the virus was suppressed without the effects of herd immunity. They are all approaching herd immunity.
Bell curves can, and do, flatten without ever reaching herd immunity, because there's a hundred factors which can impact the spread of a virus. The idea that the term "herd immunity" applies when we're still in the first wave, with recurrent outbreaks, is nonsense; that's simply not what the term means.