I have attached a very small patch fixing the value that org-attach-store-link-p gets through the customize interface. The additional quote prevented org-attach-attach from calling org-attach-store-link when org-attach-store-link-p was configured to "Link to the attach-dir location".
diff --git a/lisp/org-attach.el b/lisp/org-attach.el
index 1816a07..7ba3d72 100644
--- a/lisp/org-attach.el
+++ b/lisp/org-attach.el
@@ -105,7 +105,7 @@ ln create a hard link. Note that this is not supported
:type '(choice
(const :tag "Don't store link" nil)
(const :tag "Link to origin location" t)
- (const :tag "Link to the attach-dir location" 'attached)))
+ (const :tag "Link to the attach-dir location" attached)))
;;;###autoload
(defun org-attach ()