This is the first I've heard of that, but it makes sense. CO/DL/NW are now in a three-way alliance and I'd suspect they want to make it as convenient as possible for code-share passengers. NW is already in Delta's Terminal E at DFW and with CO there anyone connecting at the DL hub using code-share flights won't have to leave security and change terminals. (E Terminal is not connected inside security to the others at DFW.)
Some of the other hubs of the three have all gates accessible without leaving the secured area ... ATL, MSP, CLE, MEM. It makes me wonder if similar moves aren't coming for DL at IAH and DTW, and CO and NW at CVG.
If you're gonna code-share, might as well make it as easy as possible for the folks. My girlfriend traveled on the NW/CO codeshare a few weeks ago transiting MSP and reported the experience to be seamless.
My $0.02 ...
R