I think that the statement should be refined to say that it is not possible to develop a monolithic kernel based OS in ISO standard C. A monolithic kernel generally relies on passing pointers to memory between components with few restrictions. This can be problematic for some languages/compilers. However a microkernel OS that provides more structure around how data is shared between OS components can support development of many OS components in multiple languages. Even languages requiring significant runtimes like Java or C# could be used for many OS components like file systems, network stacks or device drivers.
Historically, it has been difficult to beat monolithic kernels for performance and efficiency, and through significant effort, monolithic kernel based OS's exist that are reliable enough to be useful. However, the monolithic kernel is not the only OS architecture.
While maybe theoretically of interest, it is far afield of the pragmatic considerations that underlie the paper: worked on operating systems in common use TODAY.
Historically, it has been difficult to beat monolithic kernels for performance and efficiency, and through significant effort, monolithic kernel based OS's exist that are reliable enough to be useful. However, the monolithic kernel is not the only OS architecture.