I'm just stating the experience I had, and the observations I encountered in that job that formed that opinion. To clarify I'm talking about a beginner that needs to be productive quite quickly to earn their paycheck so their code has to somewhat scale to a medium sized solution and get accepted by a senior dev in a PR process. If they aren't beginner concepts as you state why are they in most "Getting Started" templates and their use is mandatory? As an example many of the common app types use a DI framework straight away following its own standard (e.g. ASP.NET, Spectre.Console, etc) and while an experienced person picks this up quickly someone new to the language from what I've seen struggles. These are concepts I had to explain to a beginner I had to mentor at the time pretty quickly.

Most F# code bases I've seen that make it to prod don't use frameworks for those concepts even as they scale; basic functional patterns seem to scale up to reasonable sized solutions. If they do use mocking/di/etc its only to make the code more C# consumable at the edge for some other C# framework they are required to use. I'm not talking about toy projects here. My point was more that at some point with C# you will need to learn these things quite quickly even for basic use cases or find yourself writing clunky boilerplate (which most new developers will default to - the you don't know what you don't know problem). However with some other languages you may never need to until much later in the learning cycle - I was working for a very successful large business that built the whole stack without ever using some of these things. If you want to encourage people onto your platform these are some of the people you need to win.

IMO It just seemed the standard to get nice code that was prod ready in F#, even JS or Go, doesn't require as much learning's as a C# app. It's not just a C# problem mind you but from what I've experienced first hand it has scared developers away.