Similar references to the next iMac was also found in the 10.9.4 release, however today's findings point towards an upcoming iMac with a Retina screen.
The file itself has evidence of resolution options, maxing out at 6400 x 3600 pixels (3200 x 1800 as a Retina display).
The first of these resolutions indicates hexa "00001900 00000e10" is therefore a resolution scaling of ... 6400 x 3600 (probably 3200x1800 HiDPI).
Continues and is 5760x3240 (2880x1620 HiDPI) 4096x2304 (2048x1152 HiDPI), etc..
(There are other resolutions, just make the conversion from hex)
With this alone, it is not possible to guess the native resolution of the machine. This is a resolution that does not appear in the file, since it is native and does not have to be "scaled".
Knowing that there is a gap between the values of 5760 and 4096, one can imagine something between these two (probably 5120x2880, 2560x1440 or in HiDPI), but this is speculation.
Apple has been rumored to be working on an iMac with a Retina screen for years now. The MacBook Pro with Retina Display was released back in 2012, and many have been waiting for the display to come to other machines. Apple could offer both a cheaper iMac and a Retina iMac, as it does with the current MacBook Pro lineup.
Surprisingly, many expected an updated iMac at WWDC, but it looks like Apple will save such announcements for a later date.