Sure it is. Certainly in the classic sense of not having memory access isolation between all services, drivers, etc. The Linux kernel may be modular, and have some kernel threads, but it is "exactly monolithic" by the standard definition, is it not?
Thus, it isn't paranoia on the part of the developers to use a language that allows one to fairly easily see what memory accesses are occurring on a roughly line per line basis (such as C).
> Ah, and I've forgotten about another large C project switching to C++ (gcc)
That's interesting, because the gcc development seems so different from Linux's; things like the copyright assignment provisions may (or may not, I'm speculating) affect the contributor pool in a way that makes the transition more practical. In any case, compilers work at a different level, so that the memory access abstractions of C++ aren't so objectionable. In fact, I suspect hey have more data structures and inheritance possibilities that would benefit directly from it, and it's probably a good choice for them. But I don't think that necessarily translates into a reason for a kernel project to do the same.
In any case, while I disagree with daglwn's assertion that Linus was "flat out wrong" about C++, it's exciting to see projects that implement a kernel in a language like C++ (or D, or Go, etc.) to know how the language features influence design decisions and ease of implementation, to see if it really matters.