If you ride with the pressure so low there is a lot of give, it requires more effort to pedal. So you need the pressure high enough so there is little give.
The bumps absorbed by a fat tire is way less than suspension. Let's say you hit a large rock, the tire gives half an inch, and suspension gives 4 inches. It is nowhere near the same.
There are benefits of full suspension bikes, but I settle for front suspension, because it is easier to put a rack on the back or a rigid bike.
Its not about how much shock absorption your bike features.
The way you balance yourself, and therefore, the way you land/encounter obstacles is what accounts for a smoother ride. Rider skill.
If you find to be already well balanced on your bike, the rigid fork will help take advantage of this.
For the majority of riding, this is how you'll be best off. Its only during the toughest of singletrack where floating over the bumps will pair well with rider skill.