In several press conferenes, employees of NASA or private space firms have been asked if they played KSp, and some answered with "Yes".
NASA used patched conics to find candidate orbits for Apollo back in the days.
With that being said, KSP strikes the balance between accuracy and simplicity. Patched conics give a good idea how space works, without being so overly complicated that its no longer fun to deal with. And that means it makes people learn stuff they otherwise wouldn't. The mantra "Space isn't high up, its about going sideways fast" can be repeated thousands of times, and people won't get it. But when they actually try it out and see for themselves why it is how it is, they suddenly uderstand. Whenthey fall back onto the planet after launching straight up, and then finally learn to pitch over and do a gravity turn to achieve orbit.
Tha fact that orbits are always influenced at the opposite side of the burn. That deceleration (retrograde burns) lowers the orbit and so forth. What radial burns do, what normal burns do, what inclination is and why it is important, how ascending and descending nodes work and why they are important, why rendenzvousz takes so long and what you need to do to actually make it. All those things are taught by KSP and make spaceflight accessible to people that otherwise wouldn't "get" it.
The "mun" - the equivalent to the Moon" in KSP is in an equatorial orbit to make it esier for new players to get there (no launch window, no inclination change), but those things can be sued to get to the second, much smaller moon, minmus.
The concept of Hohmann transfers, bi-elliptical transfers and other transfers is explained. The difference between fuel-efficiency and time efficiency can be learned by KSP. Ejection angles and launch windows for interplanetary missions can be learned with KSP. Gravity assists work, and can be very useful. Thats a concept a lot of people struggle with, but in KSP you can try it out and see how that works.
Re-entry heating exists and matters somewhat, depending on the settings. You learn that you should neither plow too fast too deep into the atmopshere neither too low so that you skip it. The concept of re-enry is hard to grasp for some people, but with KSP, you can easily see why it works how it works and whats important about it.
The rocket equation, and especially the tyranny of the rocket equation, is improtant in KSP. You learn good engineering techniques. You learn that you can't brign what you want, that rocket grow exponentially. the concept of staging and why it is important comes naturally with KSP. Some basic aerodynamics - how CoM and CoL work, how drag affects the path of your rocket - are explained.
What Isp is and does is explained. That Isp at sea level and Isp in vacuum are different things and are important is explained. That you can steer a rocket with gimbaling engines, reaction wheels or RCS is explained, and that some of those are better in some situations then others (RCS for ascent? Not the best idea!).
In short: KSP teaches a lot, while using physics and orbital mechanics that are not so overly complicated that it gets too frustrating. it stays in the area that most people can still grasp.
And for those who want the extra challenge, the game can be modded. N-Body physics are provided by a mod called "Principia", and the star system can be transformed to the Real Solar System by the mod of the same name (RSS). Historic engines and other parts are provided by the mod called "Realism Overhaul" (RO). Signal delay is provided by RemoteTech, life support by one of the various LS mods (Kerbalism, USI-LS etc).
So apart from the base experience that already teaches lots of the concepts of spaceflight (I forgot about heating and radiators in my list above) while maintaining accessible and approachable (if you don't want to learn too much, trial and error will get you somewhat far), the modding community has created a lot of mods that bring even more important concepts to the player (FAR has a more realsitic aerodynamic simulation, for example).
And the game lets you deal with it how you want. Don't want to learn about launch azimuth? Fine, don't. The parts have enough delta-v so that you can be sloppy sometimes. Want to learn? Good! You can learn about it, and then apply what you have learned and see it actually work. See your orbit end up exactly what you wanted (or not). The community has created lots of maths tools and helpers for that as well.
Finally, Squad (the developer) already has/had a deal with NASA. NASA helped them make some parts, the so called "NASA parts" that are still in the game today (I beleieve they were originally for the "Asteroid day" official mod, but are now in the base game).
So yeah, its not a 100% accurate simulator. But its close enough to teach the concepts, and simple enough so that its accessible for a very wide audience. In that regard, I think it offers a great example for how educational games shouuld work in the future.