Definitely not the case.
Base building for running is also to promote mitochondrial development in the lower body musculature and strengthen/prepare the connective tissue that is stressed by running.
LISS running does make you a better runner than other forms of LISS will. Much better. If you want to be a better runner you absolutely must use running for your LISS.
Yeah no shit LISS running will make you a better runner than other forms of low intensity/steady-state like circuit training or swimming. That's the power of specificity. I'm not saying otherwise.
Base building itself isn't just used to become a better runner though, you can use it to benefit your aerobic system period, whether you're a runner or a mixed martial artist. If you're MMA you don't have to run for your Base, you can do other activities within the HR/time parameters to get the same cardiac benefits and improvement in aerobic system.