I have no idea about what proportion of the US Worlds team is or isn’t clean, let alone any specific athlete. I hope they’re all clean, but cynically I know better.
Like a number of others here, I can sadly say that from a number of years of D-1 level swimming in the mid to late 80s … swimming is not 100% clean.
That said, I also have personal experience of messing up the prelims of a distance event (500 FR, 400 IM, even 800) where you are rested and ready to race … and go way faster than you should have (or intended). Consider, your coach says something like " … stay relaxed, just cruise it and qualify … " and so you have no pressure, are relaxed, probably way out in your heat and the 18-19 YO competitor takes over and you feel great - so you go with it and nail a great time – only to realize you went harder than you thought and (aside from PR or Meet Record or WR) have probably burned a match in an unnecessary place. It happens … and you’re so happy it felt ‘easy’ that you tell people that (remember, you’re maybe 18 or 19) and it comes off sounding hollow at best or more likely disingenuous.