Does tea play a significant role in kombucha fermentation? I was reading about different types of fermented beverages, and it struck me that kombucha and water kefir are remarkably similar: water + simple granulated sugar + SCOBY. I'm skeptical that the addition of tea significantly contributes to or changes the fermentation process, so without the tea they seem very similar. I'm curious if a superior tea flavor could be achieved by brewing the kombucha like a water kefir (ie omitting tea) and adding a freshly brewed tea concentrate later.

Other than the cultures specific to each process, though they likely both share S. cerevisiae
You do not have to use tea to make kombucha. For example, the Noma Guide to Fermentation contains seven kombucha recipes, none of which use actual tea. A recommended way of storing SCOBYs when not brewing is to store them in a water + sugar solution, which needs to be refreshed every so often. Thus, your suggested method of making "kombucha" from water + sugar, then mixing with a strong tea would work in principle. 

That said, I would expect the result of this method to be essentially a mix of tea and acetic acid, with little to no added complexity of flavour. For me, that complexity is most of the fun of making kombucha.