Originally (at least for the wifi platform we are using), radio firmware was being linked directly into the application image. I.e. it was/is contained within the stripped elf file.
I believe support was added for storing the radio firmware in a different location at some point though (e.g. in an external SPI flash), which may be what you're using. If so, my guess is that the canned OTA process provided by the framework does not support a mechanism to upgrade the radio firmware in that alternate location; you'd have to build it yourself.
Just to remind you that if update radio bin firmware fails, the device is dead because the wifi no longer work so you cannot access the device.