Nah, it can't be the multithreaded rendering. Multithreaded rendering in DX12 has very low overhead as the workload is spread more or less evenly across all the CPUs. But looking at the screenshots that gradoman posted, his CPU usage doesn't seem very heavy at all.
Nowhere near to being maxed out, unlike DSOgaming's setup.
There are two possibilities,
You have a fixed workload like going for 30FPS or 60 or vsync,then a fixed amount of workload get's split up into smallish threads and you have the so called "low overhead" (basically you just don't need a super fast core to do it in one thread)
Second possibility you go for no vsync and variable fps in this case you have an infinite workload,no matter how much you split it up each splinter remains infinite,all cores fill up and you scrape at 100% utilization.
Gradoman uses vsync/frame cap at 60 fps at least his screenshots show only 60fps.