This statement was true in the '90s. Nowadays, the amount of high-level software written as a ratio of total programmer-hours is probably upwards of 90%.
Out of the hundreds of thousands of programmers out there, very few of them write kernels or drivers. Even fewer write window managers—there are maybe a dozen of them in active use, and most of them have either zero or one maintainers. Four software companies, out of thousands and thousands, write Web browser engines; two of them share most of their engines' code. With few exceptions, all of the codebases I mentioned date back to the '80s or '90s, when most of the code was written.
People seem to think that the software stack on a typical desktop OS is constantly under heavy development. It's not. Most of it is in maintenance mode or effectively unmaintained.
Don't take my word for it—look at the "who is hiring" threads on HN lately. There's some C++ in there, sure, but nowhere near the figure you quoted.