NavList:

The old sailor treated by Maturin had been ill and deteriorating for weeks, so likely had the even more dangerous chronic subdural hematoma. Many of these die even with modern neurosurgery, so Maturin was rather lucky I think!

No, I don't think Maturin was lucky, he's a fictional character, it seems to be that the scene would have been written to establish or confirm the skill of the character as a physician.  Success or failure was a plot point.  Likewise the later trepanning on Maturin was aborted because 2 successful operations would have been streching credulity among those readers that understand the practice.