You will need to tell us what the other person was actually talking about, because neither expression makes any sense on its own. "Tide" is clearly a metaphor, but I cannot guess at its meaning here. It is not an idiom, not in relation to rising and falling, although using "tide" to refer to a flow or movement of people is common.
I haven't read the novel but from a quick look at the chapter in Google Books, the other person (his uncle?) had previously compared the people of the city to water molecules, and the number of people coming into the city to work and going home in the evening to a rising and falling tide.
Ah, so it is linear movement then, which is the usual meaning of "tide of people". The linking of the tide of people travelling in one direction in the morning and in the opposite direction in the evening to the rise and fall of a tide is unusual. I expect most people are unaware of the link between the rise and fall of literal tides and the flow of water, so applying it as a metaphor would not occur to them.