my machine is an 27 inch imac with 675mx graphics.
Did you read the other person's post where they indicate they are running on a 2010 Macbook? I hope not because surely you wouldn't expect the GPU, etc. in their system to stand up to yours which is the highest end Mac currently available from a gaming perspective.
A working solution for a game typically needs to run well on midrange hardware to be successful otherwise, the system requirements become self-limiting for the game's sales to a particular audience, in this case Mac gamers, among whom you are in the minority with hardware that powerful.
Mine is the system that precedes yours by one model year and had the best available GPU when it was made, the Radeon 6970m. I don't think for a moment I'd get the same performance you do. Up until early this year, I had the best possible Mac for gaming.
So no, I kinda doubt GW2 just runs fantastic for all Mac users just because it happens to for you on the latest, greatest hardware. Keeping in mind the broad range of Mac systems out there and the fact that high end is the minority, it behooves game developers to try and develop with the lowest common denominator at least in mind for some sort of playable settings. Sure, not everyone with older systems, mine included at this point should expect max everything all of the time, but playable frame rates with decent visuals? Yeah. I think that is a reasonable expectation and one that I also think most game developers strive for where ultimately it is in their own best interests to provide the fun for as many users as possible.
If Windows games shipped to run well only on high end machines, most if not all of them would fail to sell well. Why should we as Mac gamers have lesser expectations for what is delivered to us and called a Mac OS X version? I don't care how it is "ported" and I use the term loosely in the case of Cider, so long as it works. It ought to work on medium settings with decent frame rates on any 2010 Macintosh in my opinion and even better on later hardware. This would more closely resemble what you might find for performance on "typical" machines in the Windows world.
Have a look at the Steam hardware survey sometime and notice what the average Windows user plays games on. They are not playing on high end rigs. The average Mac user isn't either.
As for being an API, I don't think a solution I can drop a game into at home and fire it up on my Mac qualifies as an API. No source code access required, you know? Transgaming who sells Cider as a quick and easy porting solution began life selling a modified version of Wine to Linux gamers. I think what they have now is simply the continued evolution of that Wine-based code.