A/b testing doesn't have to mean extra work. If the small startup is going to redesign their homepage anyway (which a lot of them do), they might as well a/b test it.
Based on this article, only larger changes should be done by smaller startups. But smaller startups have less resources to throw at larger changes. I mean, what of we're not planning on redesigning our web page?
That said, the kind of results startups could get by tests are usually obvious at the balance sheet (or some other big metric) anyway, without any kind of formalization. If you have one of the rare situations where it isn't, go ahead and do a formal test, but I wouldn't recommend anybody to lose some sleep over it.