How to dehydrate nuts using an oven for a short time?

Dehydration (low heat, long time) and cooking (high heat, generally shorter) are two separate things. Dehydration lowers the water content, whereas cooking changes the chemical structures in the food but may also dehydrate to a greater or lesser extent.

To dehydrate you need to gently heat for a longer time so that the water evaporates without cooking the nuts.

Higher heat for shorter times may or may not dehydrate depending on the temperature - you run the risk of burning or cooking the exterior making a soft interior and hardened shell. This may result in the interior of the nut potentially being able to grow any contaminating fungus or bacterium, which you might not be able to tell from the outside.