Technically it's an IPv4 router once you enable net.ipv4.ip_forward in step 1, the rest is enabling a whole lot of supplementary services and operations not intrinsic to the definition of a router.

I didn't see in TFA --although I may have missed it-- where it said it was replacing the ISP's router/CPE. Anything routing traffic is a router.

At home I've got both a CPE given by my ISP and my own router that routes and bridges traffic between two LANs of mine (192. and 10.).

Moreover the lack of IPv6 inside our own LANs is, for many of us, a feature. It doesn't mean we don't have an IPv6 address: it just means we have the choice and did choose to have our own LANs on IPv4 only. And, no, I don't care that it makes some programmers at some megacorp' lives more difficult to "reach" inside my networks.

I'm the boss at my home and my router is IPv4 only.

And I've got that in addition to my ISP's CPE.