I find it interesting that when talking about taking falls on hard surfaces, we always talk about the "big slap" in a binary fashion. I am either going to slap the ground, very hard and bruise my arm and or hand... or I am not going to slap at all. I am not convinced that it is a binary thing. When taking a fall on a hard surface, one of the things that you can adjust, is the power of the slap, and how much of you arm is used in the slap.
We all tend to agree that we want to maximize the surface area for the impact. Slapping with the arm, increases that surface area. The amount of power you put into the slap, can be variable.
When practicing falls on a mat, you need to learn more than just how to slap hard. One of the more important things to work on with the slap, is how to make contact with the softer bits, while protecting the harder parts of your arm, like the elbow. That way, when taking your fall on a hard surface, you can give the slap more or less power as the situation dictates, but you should be able to slap harder without issue, as you have learned to slap in a way that does not impact the harder points of the arm.
When I demo falls on hard surfaces, I choose how much of my arm impacts the ground, based on the fall I am taking. I have done no slapping, slapping with the whole arm, hand and fingers, and everything in between. (everything, but the fingers, everything but the hand, just the upper arm...) I find that by adjusting the power used and the amount of arm in the slap, I can take falls on hard surfaces quite easily with no bruising from most falls. I also have the ability to take some really hard falls and only really worry about the bruising in my fingers and occasionally elbow.
Weirdly... in the times when I fell by surprise.... as in you are walking and realize that you have slipped and your feet are above your head and you are on the ground... those falls have been the softest falls I have ever taken, and I have no idea how much I slapped at all.
Anyway, my point is that the slap is not either / or. You can vary the power used, and learn to use the proper surfaces. The more options the better.