I’ve run Zwift on a 2013 MacBook Air and it works great in the Basic graphics setting. The new retina Macbook seems to have less power than that, and as you said, it is below Zwift’s specs. It may not run at all and if it does, I wouldn’t expect stellar results.
I’m a huge Mac fan and the new MacBook Retina is a sleek piece of hardware if you want something ridiculously light with a retina display. The MacBook Airs just got nice upgrades, though, and will undoubtably run Zwift better. Additionally, you won’t need an extra ($70 or so) dongle to plug in the ANT+ stick.
I should note, too, that I mostly run Zwift on a Mac Pro which will play it on Ultra graphics mode. The difference in graphics quality over the MacBook Air is huge and I think it makes Zwift much more enjoyable. If you want better graphics in a Mac laptop, consider the MacBook Pros which will get you into “high” graphics mode.