This is misleading. Serum antibody titers normally fall after you recover from an infection...the high titers induced by the infection decrease to baseline. The implications for immunity are unclear.

Here's a quick summary of some hypothetical scenarios. https://twitter.com/ReutersUK/status/1321018815770222592

1. Antibodies drop to baseline titers that are undetectable by commonly used serology assays. There are still low levels of antibody as well as memory B cells that can rapidly produce more antibody in the event of re-exposure (anamnestic response).

This is not "waning immunity"

2. Some of the people identified as part of the 6% were false positives. Serology tests can be notoriously non-specific, so this depends on the tests that were used.

This is also not "waning immunity"

3. Some people (possibly with mild infection) do not have long-lasting antibodies for whatever reason. They do not have sufficient memory B cells to mount an effective anamnestic response.

This is "waning immunity"

It's also possible that all three scenarios could be true to some degree. Some people have IgG titers below the limit of detection, some were false positives, and some genuinely have fewer antibodies.

The bottom line is: implications for long-term immunity are unclear.

However, this should underscore the fact that "natural herd immunity" is not the way to go. There are too many unknowns to choose this as a safer, easier path to population immunity vs a vaccine that undergoes rigorous evaluation in controlled trials.