I think Biles at 7 is a bit high, she's had great success at the World games but her 4 Olympic golds is 2nd in US history for gymnastics (men & women) and her total Olympic medal count has her well outside the top 100 all-time list.
Phelps though stands at the top for most medals as well as most golds if you're talking about the Olympics the conversation for "best" starts with him imo.
A few other athletes I could see a case being made being higher than Biles from the Olympic pool specifically. Mark Spitz, Carl Lewis, Jenny Thompson, Matt Biondi, & Allyson Felix.
Also I see their list isn't US only, to have Simone as the highest ranked Gymnasts is kind of crazy. Larisa Latynina holds the record for the most Olympic gold medals by a gymnast, male or female, with 9. Her total of 18 Olympic medals was a record for 48 years. She held the record for individual event medals for over 52 years, winning 14.