It is not open source, however, the platform is very open and a large ecosystem of third-party developers has existed for decades. It is like Windows vs third-party software. Windows isn't open-source, but people can still use it for amazing projects that go far beyond what Windows offers out of the box (e.g. MS Paint vs Photoshop).
Microsoft had a brutal effect on IT through its anti-competitive practices, especially in its earlier years, when it was extremely influential. Lookup "embrace, extend, extinguish" and the memorable quote "Linux is a cancer".
I can appreciate that the simulator offers a modder friendly environment, but really that's comparing a prison yard to a forest.
