I believe that 'platform' refer to the thing the rest of the thing sit on.
For applications you can see the platform as the OS, but for a microkernel based OS, that can support many microkernels, it make sense for me to call platform, the microkernel choosed.
HelenOS sit on it's own microkernel named Spartan, just as Genode OS sit normally on one of the L4 family microkernel (or sometime directly on the hardware).
Frankly, I don't really understand the rest of your message.