That is the exact same language that already existed with regards to hit points, there's no change there. And again, it's not being read correctly. As I said before, what you're reading means that you don't look up the monster you change into, and its listed max HP becomes yours. Your maximum hit points are still based on your own hit dice (as opposed to the monster's racial hit dice) and whatever your con score becomes. If your con mod changes, your max HP changes. My position is supported by the fact that the original Skip Williams article I posted is from May of 2004, and the DMG errata is from March of 2004.
edit: as an additional note, I know this makes polymorph/shapechange even more powerful than they already are, but I don't think giving out the wrong information because it's more fair is the right thing to do.
Also, for what it's worth, metamorphosis works the same way, and it is correctly listed as changing your HP.