Yes, you are right. Messages to resource manager are sent using the powerDrown method, which is defined in CastaliaModule.cc
The pointer resMgrModule is left there from earlier versions of the code, where the pointer was calculated once in the radio module and used directly from there. With the new design, any module can call powerDrawn without having to explicitly handle pointers or messages. I thought we had some checks so that only specific modules could draw power (modules that are identified as hardware that spends energy) but I cannot find it now in the code.