Basically what these people are saying that your current motherboard, while it may work with the 13900k, is perhaps not designed for such a top tier CPU that draws as much power as the i9. The components on the motherboard may overheat, causing stability problems or causing the CPU to throttle. I cannot tell you if they are right, but I do know that the 13th gen CPUs are pretty power hungry. If you do have to change the motherboard, then that opens up the option to look at an AMD x3d CPU for you.
And no, it’s not just cache, it’s always a combination of things.
Architecture, frequency, core count (but anything more than 8 is overkill for just games, but it doesn’t hurt either) and cache, plus memory latency. MSFS is a game that benefits much more from cache than other games. Eg. the 5800X3d outperforms not only the 5800 non 3d despite having lower frequency, but also 13900k in MSFS, whereas in most other games the i9 will win. And all indicates are that the 7xx3d variants will repeat that and then some.
Disclosure, I have my eye on these too, so I may be a bit over excited for them lol