A SCOBY is not needed to start a batch of KT. Large brewers inject oxygen into their tanks to keep them from forming and to feed the yeast. A SCOBY is, however much it contains dead cells, alive. The top layer is very active, in fact.
Making dozens of batches side-by-side, some with SCOBYs and some without, I find a couple things. Both will brew nice kombucha - and SCOBY-less brewing can often produce a better brew because it takes longer and more of the alcohol is turned into acids. However, in my tests a SCOBY-less brew is less protected (has fewer of the beneficial bacteria cells you want) and will more often, under regular conditions, allow for non-beneficial bacteria to interfere. You might also wonder why the formation of a top layer at all? Is it to seal of the oxygen in the battle for resources between yeast and bacteria or simply to capture all of the escaping alcohol?
Perhaps the bacteria, which require oxygen, fill with gas and produce a raft as an evolutionary mechanism in order to allow a specific colony to thrive. It's the top layer that is most active, after all, and it's also the top layer that often doesn't touch liquid but is fed by slight moisture, escaping alcohol and gasses trapped between the top of the liquid and the container -- which brings up questions regarding the type of container (tall or wide) and the type of covering (very tight, not tight at all). I've found narrow tops using coffee filters produces a much different brew than a wide top using a cloth covering. I'm just guessing, but I believe this is because there is a battle between types of bacteria (some that live in liquid, some that need to float; some that can survive in an environment rich in gasses, some that require more oxygen) while concurrently battling with the yeast (top or bottom forming), which can switch between being aerobic and anaerobic.
You can brew KT with lots of different sugars, lots of liquids, in many containers and using SCOBYs or not, injecting oxygen or adding yeast and bacteria directly into your brew ... but it begs the question: What are you trying to achieve with your brew?
I say this because my current issue is how to up my bacteria count and slow my processing time in order to reduce alcohol. It seems that using the new SCOBY layer (rather than no SCOBY), reducing ambient heat, and directing my heat source to the top of the tank rather than the bottom will increase the bacterial activity and, given the types of bacteria I'm growing (Gluconacetobacter), increase sweetness while lowering alcohol, since my sugar is already at negligible levels.
Okay, so that got a little derailed.