The Payflow gateway is used by the Paypal Advanced system, that is the checkout method you set up. This does not use Express Checkout.
that is a completely separate API.
the code that passes info to the gateway is in the webassist/ecart/PP_PayAdv_PHP.php file.
If you would prefer to use PayAl Express, you can recreate the checkout page s and select To Rely on the Gateway for Secure Checkout, and select PayPal Express.