Reboots don't typically wipe RAM. Although wiping ram is relatively easy if you are early enough in the boot process (or late enough in the shutdown process).
I'd expect that the RAM encryption key is regenerated each boot, so the RAM should be effectively wiped when the key from the previous boot is deleted from the memory controller.
With ASLR and tons of activity happening during the boot process, it's almost guaranteed that you'll damage the keys you need. Plus, we don't know how shutdown processes are done. It might be wiping the keys clean before resetting the processor.
Typically yeah, I think you're right. But I seem to recall reading that iOS does some special stuff when shutting down/booting related to RAM but of course now I cannot find any source backing this up :/