I couldn't find anything either.
My guess would be that you aren't technically in the PhD program until you've passed your qualifying exam. Until that point, you're a graduate student but not a PhD student. After you pass your preliminary exam, you're a PhD/Doctoral candidate.
Also, it's a moot point. There are no bragging rights to be found in simply being admitted to a PhD program. Plenty of very intelligent people fail to complete a PhD program (usually, IME, because of personality issues/conflict with the advisor (sadly extremely common), inflexibility of the advisor and/or department, or personal reasons (economic, social, health)).
(to be clear, Musk didn't brag that he was admitted to Stanford - he suggested that he felt dropping out was more useful than continuing. His fans often hold up being admitted to a PhD program as evidence of his engineering background)