Youre right about this. My Expresses are used only for airplay, so the WiFi mode is either off, or set to “join a wifi network,” depending on their proximity to an ethernet switch. They are basically acting like any other device on a wifi network, and not as a router.

I have three of the latest/last Airport Extreme towers, and two of those are the only ones with the wifi turned on to “Extend a network.” Since they are all on ac, theoretically there should be no slowdown to n speeds.