I don’t think you know much about multithreading. First of all, we’re talking about CORES here, not CPUs. The main purpose of multithreading is certainly not to run other applications, but to increase performance within a single application. It is perfectly viable for a game, flight-sim or not, to use multiple threads. Sound, input, netcode, weather, instrument simulation. These are all candidates for multithreaded coding. Virtually all modern games use this. When I run MSFS 2020, ALL my 8 cores are utilized, which is what I would expect.
One core running “superhot” is not normal for a game, because in a game there is always a lot of waiting for each thread. Cores running superhot (i.e. 100% all the time) is what you would expect in a benchmarking tool.