Potential reason is that they need see how their tests affect screen burn. They mention they specifically need Mac hardware so this could be why they specifically need retina screens.
It might have more to do with running GPU-assisted tasks through remote sessions. You need to either leave a screen attached and on or trick the device into thinking one is attached and on to take full advantage of the GPU on Mac hardware. You can actually do it with any mac hardware (even headless devices like Mac Mini's and Mac Pro's) but it's not advertised well enough.
For example, with mac Mini's in a server environment, you need to plug in a hdmi enabler to get full GPU performance front the system while using it over remote access.
The HDMI dongle is a good solution for most users. It can also be worked around in software (although it is not trivial). It's kind of annoying that OS X requires jumping through these hoops, but at least there's a few solutions out there.
