I wouldn't say XDP is an alternative to DPDK. XDP severely limits what you can do as you insert code inside of the kernel. DPDK exposes the NIC to regular user code so you can do what you want. XDP is useful when you want to modify what the kernel does with packets before they go to their destination application, DPDK when you want to do whatever with the packets without sending them to any other application.
That's why the parent mentions AF_XDP. AF_XDP allows to get raw packets into userspace, bypassing the kernel stack. There users can run e.g. a custom TCP stack to process the packets. That's similar to what can be done with DPDK.
And both tools allow to export a "regular NIC" to the user. With AF_XDP one would probably just run the AF_XDP socket in a way which intercepts packets for a certain port, and leave all other traffic alone. That would then flow through the regular kernel stack, and the network interface would continue to show up as without XDP.
