The same way that glass/carbon fibers can reinforce plastic when they are mixed into the material. The aluminum (or fibers) don't need to melt or fuse to each other, they just need to be there to lend their strength to the surrounding plastic material. I would think the effect would be greater with fibers than powder but basically what's happening is that the plastic is encasing the added material such that when you try to bend or warp the plastic, some of that stress is handled by the stronger material embedded in it. Since that material is stronger it can handle more stress than the plastic could on it's own so the part becomes stronger overall.