There are no shipping adapters that take a HDMI / DVI / Display Port signal and capture that video and send it back out over Thunderbolt directly to an Apple Thunderbolt display. There certainly weren't any in existence July 2012 when that item was posted to MacRumors.
It's most likely that the reports are confusing the existence of many TB -> other adapters that are ubiquitous and relatively inexpensive.
Since we are finally seeing video capture devices that use Thunderbolt for the ingest to the Mac (as opposed to pumping out video over Thunderbolt to a Thunderbolt display) it might be some more months or even years before something like you ask gets released. Of course, I could be totally wrong and someone will release one tomorrow.
I would explain things as follows: it's very easy and requires no expensive tools to take flour, water, and yeast and make bread. It's an entirely different situation if you want to start with bread and make flour, water and yeast. The analogy is slightly strained since you are just feeding one digital video signal and looking to extract/process it into another format of digital video signal, but at present, only a very expensive video capture console is capable of doing what you ask.
The cheapest working solution I can come up with would be the following convoluted processing path:
- iMac sends video out Mini DP out
- to an adapter for mDP to HDMI
- to Black Magic
- plugged into to a Mac with a Thunderbolt monitor
- feeding live video into Video Processing software on the Mac
- that the graphics card would then show in the Thunderbolt display.
This makes no sense. You'd sooner sell your iMac and just buy a Thunderbolt iMac. If you were OK with a networked solution, you could also run AirDisplay software on the Thunderbolt Mac and send your iMac's video over the network to the second Mac, but that wouldn't be as low latency as the above convoluted solution and still involves two Macs.