About correct.
Aspyr, MacSoft and others are contracted by the original developer (or publisher?) to port a game to Mac OS, when the developer isn't doing it themselves (Blizzard and id are two of the few who do develop both versions in-house). Though it seems nowadays some developers just slap on Cider to the windows version and call it a "port" (Spore is a prime example, and I bet many of EA's recent titles share the same lazyness) though running a Windows application through a modified Wine isn't porting (it's not emulation though, since there's no faked hardware, just the Windows functions are "translated" into Mac OS functions at runtime, but since everything is translated while it's running, hardware requirements will always be higher).