Even though they call it Mac OS X, it's not as if they just bumped up the version of the Mac OS codebase.
Literal
Mac OS X [quotative-と言っても], Mac OS [possessive-の] code [object-を] version-upped-[nominalizer-(omitted)] [explanatory-(omitted)] [it's not that-訳ではない].
The key pattern is ~といっても, 'even though one says X, (it's not quite)...' — used to pre-empt a misunderstanding by conceding a surface similarity and then correcting it. 訳ではない is a standard emphatic negation, literally 'it is not the case/reason that X', used to knock down an implied assumption. バージョンアップ is the Japanese-English compound ('version-up'), widely used as a noun-verb for 'upgrade' even though the English original doesn't work this way.