I have a 2009 and 2010 iMac and the 2010 has an LED/LCD display and is far brighter than the 2009 model which has an LCD only. There is a noticeable difference in brightness when they are next to each other. So much so that the first time I had them side-by-side, I thought there was something wrong with the 2009 model, but there wasn't.
He probably means early 2009 (20" or 24") as those were not LED. I had a 24" and my 2012 iMac is brighter, even though the early 2009 one was plenty bright.
Erm... that's not really true. Yellow tint has affected prior-2012 models just as much from what I can tell.
Anyway. just taking into account the reduction in distance between the LCD panel and the glass, it's possible (given everything else remaining the same), that the 2012 LCD might appear a touch brighter due to the lamination process. I'm guessing it's probably not a tangible difference, though.