Of course we can scale our intelligence horizontally. We have almost seven billion of us walking around. It turns out scaling horizontally gets you a lot, too. You might be able to scale horizontally more efficiently if you had a bunch of brains-in-vats that are telepathically linked into some sort of hive mind, but probably only by constant factors.

We can also self-modify within certain limited parameters (learning, drugs, etc.), but that's the most any intelligence can do. Whether we can widen those parameters by more than a constant factor is an open question.