Keep in mind that driving manuals are deliberately written at a 6th to 9th grade reading level depending on the state.
Yes, you're right that the gains from speeding are marginal compared to the risk, cost, and stress. But it's hard to communicate that kind of nuance. If the manual just says lead-foots aren't going to get somewhere faster by speeding to each intersection that's "good enough" and very close to reality for the purpose of the manual. National merit semifinalists can get the queue-theory version from their high-school math/driving instructor during driver's ed.
Is it though? Almost all the knowledge we have is taught using flawed and simplified models that we can niptick and find "obviously wrong" when deeply examined. https://en.wikipedia.org/wiki/Wittgenstein%27s_ladder
And IME, when I'm explaining something, pedantry and specifics almost always get my audience confused more than help to get the point across.
over enough journeys, driving faster will always decrease your average travel time (unless you crash or get pulled over, I guess). the way to discourage people from speeding is not to tell them something obviously wrong, but to discuss the legal/safety risks and the increased gas and brake pad consumption. I understand that I can get places faster by speeding in the city, but I don't do it because, to me at least, the gains are marginal compared to the risk, cost, and stress.