I think the terms are being used incorrectly here
Yes, I must confess that I have cencentrated only on this part of the original post
When the density of the air decreases, the suction acting on the fuel will decrease and less fuel will be mixed with the air. But does the decrease in density/pressure cause the fuel coming in to decrease so that the fuel/air ratio remains constant?
I have ignored this part
Is it because we're creating less power now that we don't need the extra fuel for detonation protection?
To answer both parts we should say that we adjust the mixture lever for two reasons as we climb to altitude. These are:
1. To compensate for the fact that reducing air density tends to enrich the mixture, for the reasons that I have described.
And
2. As altitude increases, the reducing cylinder pressures reduce the risk of detonation. This enables us to stop feeding in additional fuel that is used to cool the engine to prevent detonation when generating high power at lower altitudes.