Since speed is a paramount factor in long jumping, it is clear that a slower runner has to be technically superior to the faster one in order to jump the same distance. Technically superiority could include speed at the board (as opposed to speed on the runway); better control of in-flight rotation; better extension on landing; and “better” take-off trajectory. “Better” means closer to optimal, not necessariy steeper. Let me turn the question on you: if a steeper take-off angle will enable a slow jumper to go farther, why would it not also help a fast jumper to go farther?
You are still stating as fact that the slower you are the higher you should jump. I am not saying you are wrong. I am genuinely curious to know why, in terms of the force vectors or however you want to look at it.
As I recall, research has shown that no jumpers achieve a take-off angle that is as steep as physics theory would call for. But I do not see how this is a bigger or different problem for slower jumpers. If it is, I want to know the reason.