> you can't generally mock a function
With `dynamic` and `DynamicObject`/`ExpandoObject` proxies (or even lower level System.Dynamic/System.Linq.Expression fun) you can mock anything you want to in C#.
Those tools go all the way back to the early days of Linq (and useful but somewhat broken DLR visions like IronPython). If you need to time travel even further back in the .NET stack, or if you are just allergic to/deathly afraid of the DLR as some people seem to be, System.Reflection.Emit has been there since day 1. It's awful to work with and even worse low-level experience than the DLR, but it is capable of a lot of things. If you've got an up-to-date compiler you can go the other direction and use the recent Source Generators to do all the same low-level things but this time in the context of Roslyn and at build/compile-time.
Obviously, that doesn't necessarily make it a good idea that just because you can do such things that you should do such things, but C# has far more powerful raw tools at its disposal than many people realize.
A lot of the boilerplate in DDD styles is simply a preference for it and (over-)design patterns as comfort food.
It's a further aside, but hand-written "Fakes" patterns require more up-front work but often seem to me much better than automated Mocks. I've never seen a good DDD pattern focus on good "Fakes", though, and sometimes I find DDD complexity gets in the way of good "Fakes".