The push-down pattern has the advantage of not obscuring any content so (in most instances) you can scroll down to see the page content while the menu is displayed (the IBM example your link to is more complicated and collapses the menu upon scrolling but the more common pattern is to keep the menu shown while scrolling).
The more traditional overlapping menu pattern is descended from well established drop-down menu and drop-down list patterns. The push-down style came gained popularity with the rise of small devices (phones) where it fit nicely to simply use the whole top portion of the screen for a menu.
The IBM example is somewhat unusual in that the menu displays a very large number of options and takes up a massive amount of real estate. It's a nice solution to the problem of presenting that many options but I think it would be more usable if scrolling didn't interfere with the menu display.