Two different sources say that this comes from the beginning of Heraclitus' book. For that reason, DK rightly put it as B1. Most of the other fragments are short oral sayings, easy to remember. However, this fragment has a very literary style. This fragment and B48 lead me to conclude that Heraclitus' book was a written and not an oral document.
Aristotle points to the ambiguity in the first line. He says that ἀεὶ (forever) ambiguously could modify either ἐόντος (is) or ἀξύνετοι (not comprehending) -- Rhetoric 3.1407b11. Some think this was intentional, while others think that it is just unclear. Some translations try to capture the ambiguity: "Of this account which holds forever men prove uncomprehending" (Barnes), where "forever" could modify either how the λόγος (account) holds or how long men prove uncomprehending of it.