Also wondering, why not just use DPDK?

First, "just use" is doing a lot of work in that sentence, because DPDK is much harder to use than XDP. The authors of this blog were surprised they had to do their own checksumming, for instance.

Maybe more importantly: they're not building a middlebox. DPDK ultra-high performance comes in part from polling. It's always running. XDP is just an extension to the existing network driver.

I think they are getting a lot of value from the rest of the Kernel's networking (VETH/namespaces etc talking to containers).

XDP is built into the kernel. DPDK is a huge framework that invasively bypasses the kernel and has to remain compatible as an external project.