I think those types of rumors tend to be looked at as just rumors. If a famous person were ABDL, it would be poked fun at but otherwise mostly ignored.
Miley Cyrus straight up said she was an adult baby on a talk show. Did an entire video wearing a diaper. Posted photos of herself in her mother's lap wearing a diaper. She ignored the criticisms and let her freak flag fly, so it was seen by the mainstream as a publicity stunt.
Angelina Jolie has a reputation for being into diapers and poop play. Multiple hotels have claimed she left shit stains and used adult diapers in rooms she stayed in - the types of stains you get from playing with it. One of her old security guys claimed she was into diaper play as well. Again, it went nowhere because it was just seen as unsubstantiated rumor.
Sarah Silverman wrote a book that included a lot of stories about her bedwetting problem. She discussed wearing diapers well into her teen years. Another ABDL will clearly see some of the connections she makes in the book to how they made her feel certain ways. I would honestly say her book makes it very clear she's ABDL.
In the end, unless you're caught red-handed, most people would just assume such things are baseless rumors.