The “10” comes from the specifications of the SX1301: 10 demodulation paths. The fact that we only use 8 doesn’t mean the gateway doesn’t support it.
The idea of using a 16 channel gateway to service multiple bands is only valid for the US and Australia (although it should be called sub-band then). In all other regions, a 16 channel gateway will listen on each channel twice to improve reception and make localization (positioning) perform better.