Yeah, no.
While yes they used some FSX code for various subprocesses, it is in general all new code. The graphics engine is COMPLETELY rewritten, and it was totally rewritten again halfway into the last two years.
The flight model is a complete rewrite over FSX, with the ability to process FSX information, but, even that has been removed to a large extent over time.
And over those last 2 years, they have been ripping out the FSX based modules and completely rewriting those as well. I.e., the tire friction and wind interaction on the ground, at transition, and in the air, and they are still working on further improvements to this functionality.
Your assertion that a new engine was too high of a cost for them is completely wrong. It’s exactly what they did and are doing. For example, It’s why within a couple of months of initial release, they couldn’t process FSX gauges in airplanes, even though their contract with Microsoft demanded it.
And, to your other point, the multi-threading of processes in the code was also completely rewritten between FSX and MSFS, and rewritten again halfway through.