It wasn't there, the pressure foamy 'device' . Maybe it is only in some terminals, or only added by Service guys as a preemptive measure against contact wear.
You can just add something there yourself, but be careful on the thickness, or you could damage the contacts due to much pressure. It should just keep it connected when snapped in, not press on it, after installing the back-plate. If even a small extra-pressure is applied, when pressing the back of your phone in your pocket, or else, can cause unwanted damage to the contact.
In regards to the tutorial pics, I really find it rude to the case and whatever may hide beneath it to use metal tools to snap out the safeties. I always use plastic tools specially designed for such tasks, they are really cheap and available online. This way no scratches occur and no possible damage to any loose rebel wire that may come in contact with the case's safety pins (just in case).
Took another look, the foamy thingy is there, but it is stuck on the backplate, glued. There are actually more, one for every snap-in contact.