The displayed CPU utilization is not wrong. It matches the CPU usage seen on other external tools like HWiNFO64 for instance. Game is going to display very low overall CPU utilization, specially on newest and fast CPUs. Main thread is running multicore on the CPU so the higher the amount of cores the lower load each core will drive. This will just result in a cooler CPU but with still low overall utilization. But this is still a single thread inside game. It gets bottlenecked because it drives a huge amount of tasks inside game code. Until they don´t split that into addional game threads that could run in parallel the main thread will be still limited.
Anyway CPU is mainly used at higher capacity only during initial flight loading and later on just periodically every few minutes when new scenery data is being loaded. If you stay at a location (e.g: landed with an helicopter in the middle of nowhere) you will see how CPU utilization is constant all the time and very low too. This is at least what resources monitoring data shows as of today.
DX12 optimizations are still beta but they are not wrong in terms of CPU. What DX12 does compared to DX11 is to allow a reduction of CPU calls indeed, allowing items to be processed in batches instead of one by one. However game optimizations have been currently focussed on memory usage mainly (at least that was one of the main targets of SU10).
Cheers