Seneca the Younger was a first century C.E. rhetorician. In A New History of Classical Rhetoric, on pages 176-177, G.A. Kennedy states the following about Seneca's ideal style, and the question of whether or not Seneca himself followed it:
Much of what Seneca has to say relates to style. "Speech which addresses itself to the truth should be simple and unadorned" (40.4). "If a man is sound, self-controlled, serious, temperate, his artistic ability is also dry and sober; if the former is vitiated, the latter is also affected" (114.3)...Excessive use of archaism should be avoided, as should a too-pedestrian or an overwritten style. Those who write unrhythmically or too rhythmically are equally at fault...Judgments differ somewhat as to whether Seneca's own prose style accords with his precepts. It is jerky and brittle and to some has seemed mannered and artificial. At the same time, it is appropriate for expression of his own complex personality. The word choice in his prose works is simple, sometimes even colloquial; his many metaphors are usually drawn from daily life. His sentence structure is equally simple but is constantly given "point" by an epigrammatic or ironic twist...His poetry is a different matter; he favored mythological themes that are psychological studies of violence and insanity, for which his tense, emotional poetic style is an appropriate medium.
So Seneca could follow his rules and use a concise, "just-the-facts-maam" style of writing, even though there are also times when his writing appears messy and disjointed.