It seems that Arthur responds to Marta as if she has firmly stated that the walking trail idea should be thrown out. But really, she wants to explore other possibilities. There is a difference between these two.
This is what D describes. That he interprets Marta's argument as if she recommending the proposal be promptly dismissed.
A on the other hand says that he interprets her argument to mean that walking trails would be unproductive. He doesn't necessarily think this. Perhaps she thinks it is productive, but not the most productive and this is why he thinks she wants to scrap the idea outright. Her idea of what is productive or not was not his misinterpretation.
I hope this helps. Please let me know if you have any other questions.