Very much a part of the standard. i.e. not specific to SBCL.
Think of defgeneric as the function signature and defmethod as the template specialization. Not sure why you say this is an error in CLOS. Looks fine to me.
That said, most implementations try to auto-infer the generic function metaobject when you use defmethod without defgeneric. SBCL raises a warning.
Good reads on the topic:
http://www.gigamonkeys.com/book/object-reorientation-generic...