Be careful: They did not say that C is not Turing complete. They say that the C Standard does not define a language which is Turing complete. There is a subtle

Be careful: They did not say that C is not Turing complete. They say that the C Standard does not define a language which is Turing complete. There is a subtle distinction between these two statements, but an important one. The larger point that the paper is making is that the C Standard is full of holes. Thus as a programmer you cannot rely on any compiler that implements this "standard" to do anything (even trivial things like allocate enough memory on the stack). Yes, it probably will work most of the time, but will it work always? No, well, then it's not much of a standard, more of a set of loose guidelines. If you want to write secure, performant portable code: you need an actual standard. I think this is the point that the paper is making. Better still, it is highlighting the places where the standard is poorly defined so that it could be fixed.