To quote the document you’ve provided, emphasis mine:
[When performing Direct Post] the payment form is provided by the
merchant; therefore, the merchant’s systems are in scope for
additional PCI DSS controls, which are necessary to protect the
merchant website against malicious individuals changing the form and
capturing cardholder data.
And SAQ A-EP is applicable to Direct Post merchants:
Your e-commerce website does not receive cardholder data but controls
how consumers, or their cardholder data, are redirected to a PCI DSS
validated third-party payment processor;
So, yes, your server that provides the checkout page is in-scope for PCI, even though the Direct Post sends card data directly to the payment processor. You should fill out SAQ A-EP accordingly.