As an Apple fan but also someone who is into VR, I can tell you that this is spot on right. Mac graphics cards are just too darned slow - Apple's been using mobile graphics cards in the quest of thinner and thinner machines and the one upgradable computer they had has basically been made non-upgradeable. A used 2012 Mac Pro with a third party modern graphics chip will run circles around a brand new Mac Pro with the top end graphics.
Just to be clear right now, VR requires a huge amount of performance. If you take the latest publicly available development kit (DK2), it has a 1080p screen that runs at 75hz. The consumer rift (CV1) is going to be higher than that in both aspects. VR requires a solid framerate of 75 fps. If you drop much below that, you start to get motion sickness. This is minimum framerate, not average. This is a huge requirement. Not only that but the system has to render two separate views of everything and also a lot of the optimisations that can be used for a 2D non vr rendering have to be disabled for vr - performance tricks etc.
Oculus is setting a relatively high requirement for the Rift - basically a GeForce GTX 970. That is going to be the minimum requirement for the entire lifespan of the first consumer rift. If you want to have a game or experience on their store, it will need to cater for that. Even in a year's time when newer and faster graphics cards are available with the same performance for a lower cost.
It's quite embarrassing that Apple insist on putting slow graphics cards into even their high end machines. The best graphics card you can get in the top end iMac is the M395X. This is a mobile chip with performance somewhere between an 880M and a 970M (mobile chips). This puts it at about the equivalent to a desktop GeForce GTX 960 card (a $200 card) and this has about 50-66% the performance of the minimum specification for the Rift (GeForce GTX 970 - a $350 card).
With my own personal experience, I originally had a GeForce GTX 680 in my gaming PC (I long ago gave up on gaming or dual booting my macs). This could handle anything I threw at it and despite being released in 2012, had about the same performance as the chip currently in the top end iMacs. When I got my Oculus Rift DK2, it was quite simply not good enough for anything other than the most basic of demos. It stuttered, it gave me motion sickness etc. It just wasn't good enough. I ended up upgrading to a GeForce GTX 980 even though I'd originally planned on waiting for one more generation of cards.
Oculus know what they're doing here. The requirements they've set are quite low for gaming and vr enthusiasts but might be seen as a bit high for the masses. The masses will come later though. By the time the second generation rift comes out, we'll have seen some pretty impressive improvements in technology - both on the side of graphics chip design targeting VR and VR hardware that can optimise technology - e.g. potentially foveated rendering. This will bring the affordability of VR down to masses.
I'm a huge fan of Apple products and I don't see myself ever switching from my main computer, wife's computer, our laptops, phones, networking, streaming boxes ever not being made by Apple but when it comes to gaming and graphics, Apple's products are woefully underpowered for the money. All because they've got to be slim and non-user upgradeable.
There are rumours that Apple is looking at getting into the VR game so maybe there's hope, maybe they'll stop skimping on graphics chips somewhat - they used to be way better than most manufacturers - latest model graphics cards as standard, no Intel integrated crap etc. Hopefully they'll go back to that.
TL/DR: I'm an Oculus Rift user and an Apple lover. Palmer Luckey is spot on correct with this. Macs have woefully poor graphics performance for the money. If you want to try 'proper' VR with either the Rift or the Vive, you're going to want to build/buy a Windows gaming PC. VR is amazing and really fun so I highly recommend you give it a try!