A lot of the color correction happens when you change the white balance. From Chris's pic, you can clearly see that the paper has yellowed. Whether or not it is as bad as his camera/lighting shows I'm sure is up for debate. However, you can also tell plain as day that Heritage adjusted the white balance using the background as 'white'. Their pic doesn't show any yellow-ing what so ever. 'Correcting' the WB removes the yellow-ish color cast from the whole poster, including the rest of the colors -- and hence makes them a bit more vibrant and seemingly more saturated, even if they didn't actually bump up saturation.
To do it properly you need a true white card that you photograph in the same frame as the poster, then correct WB in post using that known quantity. Some places actually do a full color profile (MPGrading) using a color charts at the side of the poster. However, that seems complicated so I'm not going to bother when I get my photo setup back up and running. Properly adjusting WB I've found is a must though...