I'm not sure I believe this claim. First off how much learning is required to subjectively solve problems in the most elegant way. Syntactically languages are essentially the same. Its the vast API's and language paradigms that need to be consumed and understood at a deep level in order to solve problems in the most elegant way with that "tool."
Now another facet of this argument is what it means to support and maintain everything. A homogeneous system is far easier to support and maintain. Stemming from your argument, a concern would be a mishmash of solutions to support and maintain. If devs were allowed to "use the best tool for the job" for every problem, I would hate to see what their production environment looks like.
The comparison of languages to a hammer is specious. A language is a language like french or italian. A tool for expressing yourself. You can translate word for word which sounds silly, or you can really "know" the language and translate concepts into the paradigms of the other language.