Well, this is referring to pre-configured NTP servers:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/opensprinkler_server.cpp#L2146
previously it’s using just one, now the firmware can cycle through an array of servers if one fails. The user-configured NTP server is still just one, because this option is rarely used. If you want to customize the NTP servers using your own, you can modify the firmware source code and compile a new firmware.