Yes: most customers don't think about the question you're posing.
For that question to even arise in a customer's mind, they would have to know what an iframe is. Otherwise, it's just a cool little popup that the website opened.
This button is the first step for stripe to start being a trusted name by more skeptical e-customers.
Of course I realize that 'normal' people might trust anything, but it still won't make me trust it any more. So I'm not quite sure in what situation I would personally ever be able to use these nice Stripe button payments.