I think prequalification is based on Amex' beliefs about your income and lifestyle, NOT your credit. I could be wrong.
In my limited experience, being prequalified for a card has no bearing for whether I'm approved, it just may give a better SUB offer.
However, if you click to apply for a card when logged in, if it says "you're preapproved!", that seems solid.
I didn't attempt this until I already had a corporate card set up, so I don't know how it works if you don't have an Amex card yet. But long story short, "prequalified" is unrelated to whether you'll be approved, "preapproved" is pretty reliable. JMO, good luck!