I have to agree with Hexstream here (2 downvotes really?). A pithy overgeneralized saying proves nothing. Nearly every occupation benefits from a worker who can learn fast: lawyer, doctor, architect, CEO, salesmen, stock trader etc...
This is not specific to programmers and it is imaginable that a slow learning programmer with domain specific knowledge has value.
The most important aspects of a great programmer are:
The ability to learn what users need.
The ability to learn how a how a system works.
The ability to learn the cause of a bug.
The ability to learn from mistakes.
And the ability to solve complex problems.
Domain specific knowledge has some limited value but flexibility of thinking is far more important.
This is not specific to programmers and it is imaginable that a slow learning programmer with domain specific knowledge has value.