Research into programming languages says that lines of code written/day tends to be surprisingly consistent. But the amount of functionality embedded in that code varies widely depending on the language.
Faster languages are ones that take over basic mechanics for you (eg memory management), infer things for you (eg compare C++ to go), or offer more convenient abstractions (eg compare scripting languages to C or Java). The difference is usually an order of magnitude, but can be more in the right circumstances.
OK so far...
> In particular some languages are inherently faster than others.
Which is an example of deciding the same class of problems, just doing in in a "different way."
So your example is not providing a counterexample to the claim; it's providing an example.
Beyond a theoretical consideration, in practice there is much more than preference providing the difference between languages. If they were the same, you shouldn't have any trouble implementing combinators in brainfuck.
Maybe the comparison of a classical computer vs. a quantum computer is more apt. There are many problems which they can both do, but for some a quantum computer is infinitely more efficient. For some linguistic problems, a certain language may be much more efficient.