3D Post Production Work Flow

[Clyde DeSouza]

... just keep in mind that if you're working at 4k in 3d you HAVE to use a beamsplitter. Otherwise your parallax is going to be excessive and you won't be able to show anything in 4k (unless some small 4kTV's come out down the line, or you're shooting massive landscape stuff where everything is far away, like in helicopter shots.)...

I'm not quite getting this. Why does shooting 4K "have" to be done with a beamsplitter versus shooting 2K on say an Epic or Red1?

From what I'm understanding of the above quote, it can't be that the physical size of the camera is being talked about here (which is true if you put two Red1s side by side for instance, you will have a large I/A)...but in that case, so would shooting 2k with the same RED1s (or indeed Epics).

So i'm inferring that what's being talked about here refers to the "size" of a 4K image?
If you finish a 4k stereo film, and scale it down with appropriate boxing / reframing, why should the parallax be affected?

To put it another way, why don't we "Have" to shoot with a beamsplitter if we are shooting in HD or 2K?

I'd like some light shed on this.
Or if something completely different was meant in the quote, do expand.

Kind Regards