In this context I'd interpret enterprise to mean any company with over 1000 employees that is not what Silicon Valley calls a tech company. So no Apple, Google or Uber, but the notable players of basically every other industry, from A for accommodation or accounting to W for wood paneling.

But I wouldn't say their choice of C# is due to them being old and boring. If it was that, they'd use Java (as many do). In my eyes choosing C# signals to me that you do want good technology (again, you could have gone with Java), but want that technology to be predictable and boring. A decent rate of improvement with minimal disruption, and the ability to solve a lot of issues with money instead of hiring (lots of professionally maintained paid libraries in the ecosystem).