Hello Tristan,
OPU3 & OPU5 settings come into effect only for planned orders & production orders scheduling.
The provision of scheduling in routing is for the analysis purpose as well as to copy the time required for production to the respective material master against 'In-house production time'. The scheduling in routing is independent of any order type.
Go through the below link to have clear idea about this.
Scheduling Routings - Routings (PP-BD-RTG) - SAP Library
Mangesh S