They are different types.

  ? (type-of "this is a string")
  (SIMPLE-BASE-STRING 16)
  ? (type-of #P"/this/is/a/pathname")
  PATHNAME

You can't perform string operations on a pathname.

  ? (subseq "This is a string" 5 15)
  "is a strin"
  
  ? (subseq #P"/This/is/a/pathname" 5 15)
  > Error: The value #P"/This/is/a/pathname" is not of the expected type SEQUENCE.

You can perform pathname operations on a string, but only because the string is automatically converted into a pathname first.