I agree "leverage" is further down the spectrum of being prone to meaningless usage (than most words), but I don't think it is as far as "synergy".

It's a more precise concept: put x in, get more than x out. The more you get out for each input of x, the greater the leverage.

Synergy is the idea of "greater than the sum of the parts", which is a useful concept but much more nebulous, in my view.

Not that this is a particularly important debate :)