That's true, but there's still a valid reason to test things on small traffic: you don't necessarily need to have statistical certainty on every decision. A lot of the time, it's sufficient to know that when comparing two dramatically different options, you can't quickly and easily distinguish between them. That's a strong hint that you should put your future efforts elsewhere. It makes no sense to dedicate lots of effort to a project if the impact is statistically indistinguishable from noise.
Moreover, the changes that matter early on are the 10x changes, and they'll often be subjectively visible to you, even if the math shrugs and tells you that it isn't confident. So while it probably isn't worth testing button copy or images on 100 visitors a day, it might be worth testing complete redesigns, or entirely different value propositions.
10x changes are hard to come by, even at an early stage. That's like going from 3% to 30% conversion. If your site is mostly broken and one change takes you to 30% conversion, then you probably didn't need an a-b test to figure it out.
More realistically, when you're an early stage startup, is that 10x means going from 0.1% to 1% conversion. That kind of change can be achieved by changing pricing models, changing product descriptions, changing the purchase flow, etc. And you'll notice it -- 1% means getting one buyer about every other day on 50 uniques; 0.1% means getting a buyer every three weeks. You won't need statistical validation to tell you that things have changed.
But even "smaller" changes -- 10%, 20%, 30% -- are usually noticeable, even with a tiny stream of traffic. The point is, the kinds of winners that you need at an early stage are usually big enough to feel. Rarely do you get to the win in single-percentage increments.
Moreover, the changes that matter early on are the 10x changes, and they'll often be subjectively visible to you, even if the math shrugs and tells you that it isn't confident. So while it probably isn't worth testing button copy or images on 100 visitors a day, it might be worth testing complete redesigns, or entirely different value propositions.