Comment by gravitas

I was curious as well, turns out they're using a non-standard parsing with a comment in the function explaining why:

    /* This uses a non-standard parsing (ie not inet_aton, or inet_pton)
     * because of legacy choice to parse 10.8 as 10.8.0.0 not 10.0.0.8
     */

src: https://git.kernel.org/pub/scm/network/iproute2/iproute2.git...

(the entry point to start tracing down to the above inner function is right around here: https://git.kernel.org/pub/scm/network/iproute2/iproute2.git... )