Thank you, as always, secondcor521.
I've done a bit more research and, if I am understanding correctly, this "six-month rule" applies to people who are continuing to work after turning 65 because, apparently, Medicare Part A (hospitalization) is automatically retroactive six months. Since DH does not work and will be applying for all Medicare to begin at age 65, then he won't be affected by this.
Could they possibly make it any more confusing? All I want to do is continue to max out our HSA contributions to the extent possible but don't want to over contribute.