Hell, I'd argue good writer doesn't equal good roleplayer.
Correct. However, you can be both a good writer and a good roleplayer. Personally, I view myself as both as I prefer collaborative writing to solo writing. I would not be immersing myself in this hobby for nearly twenty years if I just wanted to write a book; the sentiment is absurd. On a semi-related note, I would also prefer to work with someone similar to my level of descriptive detail, narrative, and characterization ability as I am rarely interested in adjusting my writing level down as it is unconstructive for me to do so.