I think it's less bullshitty, if you factor in both the experience of the language user and importantly the nature of the problem that you're trying to solve.I call bullshit.
As long as the developers are experienced with the chosen language, it will not make a lot of difference in development time.
There will be some small set of problems where it will be quicker for an experienced author to solve it in C++ rather than C#.
e.g. "refactor this collision detection system so that it generates no more than 5 cache misses per frame".
I'd say (IMHO) this kind of task is more suited to C++, but if the code was in C#, and I was a C# expert, then I'd be able to use explicit struct layouts, etc, to achieve the same results in a similar time.
As usual, generalizations only give us a general idea of the truth.
It depends on the problems being solved. I use both languages professionally -- about half my code is C# and the other half is C++.
So bottom line, does it take more time or not??
For the half of the code-base where C# is my chosen language, I do find it to be much less taxing on my brain energy...