What Stripe does is a gray area, but something you can be pretty sure the PCI Council will eventually clarify so that these things are clearly covered. They may or may not be now.

Why will it be? Because whether the form submits to your server or to someone else's server is irrelevant. If you have not secured your own server, then someone can redirect the target of that form, or inject their own Stripe-style code to silently send the form contents offsite the same way Stripe does it. And nobody will be able to track down the source of that breach because the site owner is probably not complying with the access control and logging requirements either.

Do you really want people that can barely configure WordPress responsible for the security of your credit card number online? That's the effect of not putting this practice under the scope of PCIDSS. That's probably why the banks underwriting Stripe told them to put the requirement that all their users comply with PCIDSS into their terms of service.

If you want to be outside the scope of PCIDSS, then don't host payment forms. Either redirect to a hosted page, or embed an iframe. Either way, being on a different domain means your own security practices can't compromise the payment data.