Loose is a shorter word than either release or discharge and 'let' would have been four extra characters (word + space). In the older games, dex entries had to fit in two tiny text boxes so I imagine their usage of loose as a verb is a holdover from when they had limited space. Gamefreak really likes using some variant of the phrasing 'loosing electricity' when describing electric types:
Pikachu:
FireRed/Y: It has small electric sacs on both its cheeks. If threatened, it looses electric charges from the sacs.
Pearl: If it looses crackling power from the electrical pouches on its cheeks, it is being wary.
Raichu:
Pearl/Y: It can loose 100,000-volt bursts of electricity, instantly downing foes several times its size.
Elekid:
Blue/Red Rescue Team: I wave my arms in circles to loose electric attacks!
Electivire:
Platinum/Black/White: Heedless of enemy attacks, it closes in, shoves its tails onto the foe, then looses high voltage.
Magnemite:
Pokemon Ranger: Magnemite looses electricity to attack. It can also recharge your Styler.
I'm pretty sure the only reason I even know this usage of the word loose is because I got confused playing FireRed as a kid for the same reason lol