It means that there is no further information on self-reference so we try instead Etymonline that has an entry on meta- (https://www.etymonline.com/search?q=meta-), but not on meta as a single word.
At this point, it is evident that meta- and self-reference depart because the notion of self-reference may not be an essential part of the meaning of meta- for the reason that not many substitutions of meta- with self-referential will bring about meaningful expressions.
In contrast with this, meta- can actually be traced back in history in a way explaining how constructions like metamathematics, metacommunication, meta-analysis (metanalysis*), metapolitics, metaphrastic, metamorphize, metaphor, metathesis, metamorphism, or metamorphosis have come up.