It's unfortunate the criterion of circulating antibody levels is being either misunderstood or deliberately misused in some cases. With a natural infection, symptomtic or not, the immune systems"sees" a large number of different surfaces of the virus. There is therefore a large antibody production response and a high titer of circulating antibodies. However, only a relatively small fraction of these antibodies can block the interaction between the spike protein and the Ace2 receptor, and thus prevent a second infection. The circulating levels of those so-called neutralizing antibodies will drop over time, at which point offsetting a second infection will require "waking up" the immune system memory cells to produce those antibodies from scratch.
With the vaccine the absolute AMOUNT of circulating antibody after the second booster may not be as high as natural infection, But the FRACTION of those antibodies that can block the interaction between the spike protein and Ace2 is higher, since that is the only target the immune system is seeing. As with natural infection, the level of those antibodies will decline over time as well, at which point antibody production will require waking up the memory cells. But studies have shown that a very high fraction of the resulting memory cells arising after vaxx can produce potent neutralizing antibodies. Which is why the vaccines are working so well at uncoupling cases and deaths in high vaxxed vulnerable people. Even if a second infection gets established eventually that sweet immunity kicks in and takes care of things. The same thing happens with natural immunity too, but the spectrum of antibodies that can produced upon re-infection is more hit and miss.
Anyway the total antibody concentration stuff is being misappropriated to argue natural immunity is better than the vaxx, which is just kind of silly. How long circulating antibody levels can be sustained at levels necesessary to directly prevent re-infection in either case is subject to lots of variables. And with a high replicating variant like delta keeping circulating neutralizing antibody levels high enough to prevent any sort of viral propagation is going to be really challenging, no matter how you prime the immune system, especially with a super sensitize detection method like PCR.