The point is that with a JIT the runtime is executing as little as possible, with the rest being dynamically optimized machine code. So this isn’t really a fair thing to say…

I could create a sample program that would be hard to make as fast in C or C++, with the constraint that the code needs to be dynamically loaded at runtime.

In practice, it's easier to modify a more dynamic program and be sure you're not breaking much. Highly tuned C tends to bake in assumptions about memory lifetime, layouts, call graph, etc. The flexibility to modify is what gives you the ability to target higher level improvements in a shorter amount of time.

Given any concrete implementation, after it's been designed in a more dynamic language, you can convert it to something fast in something like C. But starting from C is probably not a great idea. And not everything can be written multiple times economically.

E.g. the .net GC was prototyped in Lisp, and initially converted to C++ with a translator.

That’s the keyword here.