I don't see anything on the landing page either, but this text from the "overview" page of the documentation seems to imply debit:
> When an issued card is used to make a purchase, an authorization request is created. If approved, the authorized amount is held in reserve from your account balance. [...] The merchant then captures (clears) the authorization, at which point a transaction is created and the held funds are deducted from your account.
This implies debit as the underlying mechanism for completing the transaction, however one could presumably build a credit-based business using these cards. If XYZ company issues a Stripe card under a credit model, then XYZ company would be footing the bill using their Stripe balance, and the cardholder would then have a credit balance with XYZ company.
I think they were referring to the little card animation on the side, that shows a card with Visa debit at some point. (Note sure if that also answers backend issuer. There's also one with Mastercard in the animation.)
Mostly, as a user, the main thing that I'm looking at is interchange revenue sharing (the main thing that makes a difference). This can vary for personal and business cards and can be a non-trivial revenue source. If this is not competitive, having the well-known Stripe integration convenience doesn't go that far. I'd rather suck it up and work with an old-school CC vendor if this means the company gets a better interchange rev-share.
Landing page have multiple cards of different types, including debit and credit.