No, as other's have pointed out it's not. There are plenty of other areas to use it, even in other game engines. OP is just trying to make it seem funny by making the exaggerated narrative that it's the only use case for C#. If Boo was still around in Unity this joke would been accurate with that, don't think that was used anywhere else
While true, businesses have it even harder to migrate to Linux (what else is there when talking enshittification?) than private users. Windows and dotnet won't go anywhere anytime soon.
On their desktops, sure. But most apps are web based and back end apps are all services - running on Linux. I worked at a fortune 100 financial firm a couple years back. Hundreds of .NET apps, all running in Linux containers on Amazon ECS clusters or Lambdas.