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 :)
Again, I'm really not trying to put the author down here - but "leverage" is a term that can very easily and often is used in a mindless and vague way. Not always, of course, in the same way that "synergy" can absolutely be used properly.
Still, it would be extremely easy for me to mindlessly posit whatever I want as "high impact", or at the very least "minimal time investment". "Minimal" and "high impact" are inherently subjective phrases (minimal compared to what? high compared to what?) and without a baseline are effectively useless.
So, I guess I still don't see a huge difference between "synergy" and "leverage" in terms of vagueness. And if vagueness exists, mindless usage is pretty soon to follow.