Even though C is used for those large successful projects, even if it isn’t wrong for them, and even if the projects grew using C through a series of justified decisions, C can still be suboptimal for them.
There is no clear explanation of why he thinks C is suboptimal - his statement implies to me that there are other languages that are clearly better for writing things like OS kernels etc. and I'm not that has been demonstrated yet.
What about the linux kernel? Or GCC? Both projects are on the order of millions of lines of code. The author's claim is simply not true.