I was trying to do the same thing, disable extended SSID on IAP to configure a mesh network, and kept coming back to this article - wanted to update with what I did in case anyone else is in the same boat.
Logged on to the AP (master, not slave,) via the CLI.
conf t
no extended-ssid
end
commit apply
Rebooted the APs, then they came up as mesh portals and I could get the mesh up & running.