Yes. You really don't want your real time threads to have anything to do with the OS after startup sequence. Significant part of the project was learning about various ways a piece of code can be preempted.
Once you get it done, it really is all really nice and predictable. And also a fire hazard because you have disabled things like SMI interrupts that are used by the CPU to figure out if it is overheating...
Every respectable HFT shop does that. What you can't do is achieve <5us execution latency at 100% percentile. You'd have to disable LAPIC interrupts for that which I doubt they did.
I don’t doubt it, but from my reading of OP’s post, it sounds they were skipping most of the kernel and OS.