There is certainly nothing new here, as far as a senior U.S. government official, such as a president, or even one of his staffers, hiding his health status from the public.
Here is an article about
presidents, from Cleveland through the modern ones, making the effort to hide a serious health issue they had from the country:
https://www.scientificamerican.com/article/a-brief-history-of-presidents-disclosing-or-trying-to-hide-health-problems/It notes that Wilson, FDR, Eisenhower and Kennedy took similar actions.
It suggests that a common motivation for doing this, whether realistic or not, was the president's fear that public perception of his being ill might somehow have a negative impact on his effectiveness as president. For example,
"The nation was in a recession, and Cleveland feared that his economic plan would be doomed if the public knew that his doctor thought he could have cancer."Whether or not Secretary Austin had a similar fear, that his effectiveness as head defense official might somehow be compromised, or his leadership of the department perceived as in turmoil, and he took measures to disguise his situation, is anybody's guess, but our history shows several of our leaders skittish about
acknowledging their health issues to the public.