In testing on my machine with WorldPay as the default commerce gateway and PayPal enabled, Web Invoice Pro is not requiring me to sign into the site to pay the invoice with WorldPay. Whether PayPal is enabled or not should not matter.
With WorldPay the user must provide the customer and billing information (but not the credit card) on your site because we must provide that information in the post to WorldPay, but that is not the same thing as signing into the site. The user then provides the credit card info at WorldPay.
The difference is only that with PayPal the user does not have to supply the billing info on your site, PayPal returns that data to us, but with WorldPay we must supply that information in our post to their site so we have to collect it up front.
Hope that helps,