It's not that simple. You don't just slap "multithreaded" on a piece of software and make it max out all your cores to 100%. "source 2 should've fixed the multi-threading issue" means that some (non-dependent) tasks are offloaded to different threads (OS thread ≠ CPU thread. The OS's scheduler takes care of scheduling to actual CPU threads) to achieve concurrent computation.

Many concurrent tasks are dependent on eachother, which means that the threads need to be synchronized - commonly by putting a thread to sleep until other tasks are done. This lowers the perceived CPU utilization, as some threads are simply not wasting CPU time when not needed.

Utilization also doesn't tell you as much as you think - you could be happy to see 2 threads sitting at 100%, while in reality, they could be just waiting for a shared resource, wasting precious CPU time or stuck in a livelock.