You're on the right track toward the end. It means they built their MVP in Python, and they've built more & more features on it in Python. Now they're looking for someone who will happily work on their existing codebase. They want it to be the best Python it can be. They want someone who knows the language inside & out; someone who goes to Python conventions and follows all the big Pythonistas on Twitter to get all the latest tips & tricks. And, they don't want daily requests to rewrite the whole thing in some developer's favorite language.
It wasn't that long ago people were clamoring for all the PHP in the world to be rewritten in Python. Ten years ago, I was part of one of thousands of efforts across the globe to rewrite a legacy codebase in hot new J2EE awesomeness. Around the turn of the century, it was a push to replace ancient COBOL with hip, modern C++. There's always a hot new language that makes the old hotness look ridiculous in comparison. This industry is like the fashion industry in more ways than we're willing to admit. But it's totally possible to run a successful business on a language with warts. Yes, even PHP.
If you ask them, they'll probably tell you they really do want someone discerning, someone passionate about programming languages. But right now they've prioritized their business needs over a rewrite for the sake of a rewrite.
It wasn't that long ago people were clamoring for all the PHP in the world to be rewritten in Python. Ten years ago, I was part of one of thousands of efforts across the globe to rewrite a legacy codebase in hot new J2EE awesomeness. Around the turn of the century, it was a push to replace ancient COBOL with hip, modern C++. There's always a hot new language that makes the old hotness look ridiculous in comparison. This industry is like the fashion industry in more ways than we're willing to admit. But it's totally possible to run a successful business on a language with warts. Yes, even PHP.
If you ask them, they'll probably tell you they really do want someone discerning, someone passionate about programming languages. But right now they've prioritized their business needs over a rewrite for the sake of a rewrite.