While Amex pulls experian for hard pulls, that isn't true for soft pulls. They soft my TU monthly and have never softed my EX. So if EX is the only one you froze, they may be softing one of your other CRAs and that may be the basis for preapproval. As for the specific email you got, it is hard to tell. If you go to their web site and go to "view my offers", do you see anything? Anything on that page is pretty solid for approval.
Be careful with amex emails. I get an email from them at least once a week which is just advertising because I signed up for their offers. A lot of people come on here asking if those are "Special Offers" for them, but almost everyone gets the same exact email. I know they're not special offers for me because I definitely don't qualify 
Like the previous poster said, check their "View My Special Offers" thing. That will either (AFAIK) soft you, or check your most recent soft from them (if they've softed you before), and it will give you a better answer.
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