I think the symmetric delay here makes sense, since you're trying to simulate the latency of a (likely) symmetric connection between another client and the server.

If you only add latency on the transmission, then 2 actions taken at the same time by players with different amounts of physical latency will be processed at different times. If we assume the game clocks on each client and the server are in sync, then this creates a competitive advantage for the player with lower physical latency. For instance, if we have a timing battle (ie: Zhonya's hourglass), where both players know that at time 'T' they must each take an action, and the first to do so comes out on top, then the player with higher physical latency is at a disadvantage.