It's more descriptive, but somewhat incorrect.

The common definition of a process virtual machine is that it's an interpreter that can be written to that essentially emulates an OS environment, giving abstracted OS concepts and functionality. This aids with portability. Another concept of virtual machines in general is, for lack of a better term, sandboxing. You're limited to only the functionality that the VM provides.

C goes halfway with that. You generally don't need to care about most OS operations if you're using the standard library(which abstracts most OS calls), but you definitely do need to care about the underlying OS and architecture if you're doing much more than that. Also, simple C definition doesn't allow for threads or IPC, both of which are provided by the POSIX libraries. You're also allowed to directly access the ABI and underlying OS calls through C.

The best example of C not really having a VM is endianness. If C had a "true" virtual machine, the programmer really shouldn't need to be aware of this. But everyone that's written network code on x86 platforms in C knows that you need to keep it in mind. Network byte order is big endian, but x86 is little endian, so you need to translate everything before it hits the network.

EDIT: I think LLVM is somewhat of a red herring in this context. Realistically, unless you're writing straight assembly, there's nothing stopping anyone from writing a VM-based implementation for any language. The problem with C and the other mid to low level languages is that if you're writing the VM, you need to provide a machine that not only works with the underlying computational model, but also provide abstractions for all the additional OS-level functionality that people use.

So C could definitely become a VM-based language, especially if the intermediate form is flexible enough.

The term "virtual machine" is already pretty overloaded. This isn't referring to virtualized hardware in the VMWare sense or a language/platform virtual machine in the JVM sense. Rather, it's talking about how C's abstraction of the hardware has the Von Neumann bottleneck baked into it, so it clashes with fundamentally different architectures like the Burroughs 5000's.

For the same reason that if you put a hunk of chocolate in the oven and called it "hot chocolate," people would be confused that it's not a warm beverage of chocolate and milk.

That is, the phrase "virtual machine" is usually assumed to be the name for a piece of software that pretends to be some particular hardware. It is less commonly used to mean a "virtual machine", that is, not a noun unto itself, but the adjective virtual followed by the noun machine.