That was my bad, I thought I had removed the submodule linking to the BSP as well. Thank you both for your help through this debug.
Ultimately, I have come to the solution of the problem, so I’ll document here my findings for future searchers.
FreeRTOS requires the Compile Options and Compile Definitions as used by your BSP.
My initial thoughts of adding the compile options and definitions to the CMAKE_PROJECT_NAME target did not propagate to all the children the way I was expecting, and thus the error.
To fix this, I added the compile options and definitions directly to my BSP target, as well as the freertos_kernel_port target. For whatever reason, adding the options and definitions to the freertos_kernel target caused an error as well.
I hope this helps future visitors.