That is the most important meaning of 'się', but it's not the only one. You can take a look here. I'd say, that from immery's post, that is simply number 4: 'się' is part of the verb without a particular reason, and the fact that a verb 'wydawać' exists on its own is more of an accident, because it doesn't share any meaning with 'wydawać się' at all.
Take a look at "śmiać się" (to laugh). I don't see any reason for it to be reflexive, but it is - moreover, there's no verb 'śmiać' at all.
The Wikipedia article linked by mihxal is very interesting as well, and I'd say that 'wydawać się' should go in the "Inherent" section.
Just as poludnie means south and polnoc means north, many words are inter changeable. Those two literally blew my mind, I never knew the directions in Polish. Most of our learning in Polish school was centered around Cathechism. It was after all run by the local church with nuns.