Yeah, but it is only in the very artificial legal sense in which Germany did not exist until she was united by Prussia in XIX (while Austria lost the same attempt).
Same way, Greek nation came into existance in XIX c. The fact is that Silesia was part of Poland in middle ages for a short time and that Slavic Silesians became Germanized by the spontaneous cultural process. And before they were as Polish as they were Czech.
Nationality and states are obviously different matters. As we all know Germany and Italy didn't exist as states before mid XIX century but of course German and Italian nations (as a cultural commonwealths, NOT in the sense of "modern" nationalism linked to the concept of blood bonds) existed hundreds of years before that happened. Having said that by no means Silesia can be considered German since the XIII century in any other sense than lingustic, because many of its inhabitants (Austrians, Poles, Czechs, Dutch, Scots) used it as their first language.