By "Typical 2K and HD digital movie cameras" I guess they mean small sensors, which lead to large depth of field due to the laws of optics (don't ask me the details, I must have slept through that class). Larger sensors allow smaller depth of field which blurs the background (or just lets you show off your bokeh), allowing the eye to naturally focus on the subject.
That's the part I don't understand: my run of the mill Canon DSLR (from 2010 though) can shoot at f/1.8 with a $99 50mm lens. the iPhone can shoot at f/2.0. Nikon D90 from 2008 could do HD video and have a small dof.
The difference between the RED cameras and your DSLR is that the RED camera is doing it at 24fps or higher with its ability to shoot 120fps. Your DSLR might be able to do 8-10fps in burst mode.
There is a huge difference between in video from a DSLR and a RED camera. DSLRs can take a 5K+ still image topping out at 8-10fps, yet only records HD video. DSLR video is a heavily compressed 8bit 4:2:0 H.264 video with ~50mbps data rate. RED cameras record 4K+ video in a RAW format that is only slightly compressed and can do this up to 120fps. Comparing a DSLR to a RED or other RAW format camera is not being fair.
There is a reason a DSLR is priced < $5000. They are great for what they are, but people are not being honest if they consider a DSLR at the same level as a RED or any other higher end digital video camera.
Of course there is (or atleast was) a difference between a RED and a DSLR, my question wasn't really about that, it was about the dof claim. Note that Canon 5D has been used in number of feature films, even The Avengers[1].
I think the idea is that larger sensors/image area allows for higher focal length lenses, which naturally have smaller depth of field. i.e. medium format hasselblads had a normal lens of 80mm, with a narrower DOF per f stop, whereas 35mm cameras had a normal lens of 50mm, with a larger DOF, etc. There's some curve somewhere that describes this.
