Most iLOMs are some small/cut-down Linux distribution. Sadly often woefully out of date to begin with (both in terms of kernel and userspace apps) and often prematurely abandoned in terms of firmware updates from the OEM. Of course the hardware is just novel enough to make rolling your own install/updates from a standard distribution impossible.
> The operating system used in the iLO firmware is a real-time operating system called Integrity developed by Green Hills Software
Green Hills Integrity has a pretty good reputation in the embedded space. I imagine this (and other iLO vulnerabilities) are in HPs own firmware modules and interfaces and not in the OS itself.