If it is in the IP/TCP layers then moving that to user-space does not, by itself, necessarily reduce latency, it merely shifts it elsewhere. If the latency is due to kernel user land memory copies then that is a different matter.
Consider, for example that the dominant costs are things like demultiplexing and security checks. If you choose to implement multiplexing with virtual network cards then you get true 0-copy multiplexing, which is much faster than the software equivalent. And many of the security checks can be eliminated by using some combination of packet filters and logical disks. (The security BTW seems to be one big difference from RDMA, which might be an alternative, but I'm not really an expert.)
Some things can't be sourced to the hardware, like naming and access control. But that's fine.
(NB, I'm not arguing for this paper's position necessarily, I just thought it was interesting, and the motivation was good enough to start me thinking about how I might get around the kernel.)
Pushing more of the stack into hardware is probably a good idea for single-tenant datacenters that can deploy a lot of e.g. Redis appliances, but those of us just renting capacity in the cloud are going to suffer from Amdahl's Law if you can only accelerate the part of the system adjacent to real hardware NICs.