Not true. The komucha scoby eats the sugar. It doesn't care what kind of tea it's diluted in.
Rooibos makes a really good kombucha base. A friend of mine makes it but I am too lazy and often buy GTs or other brands.
I am extremely caffeine sensitive and, due to a seizure disorder, I am under neurologists orders to be completely caffeine free. That said, I can and often do drink kombucha as a bedtime drink and it causes me no problems at all. If anything I feel like I sleep better with it.