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.
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 :)
Synergy was frequently used in a mindless, vague way by individuals who wished to appear in-the-know. Thus, it was devalued, as people actually in the know did not want to be mistaken for those those who didn't know how to use the term.
This dynamic usually happens with high-level abstractions that have much complexity and nuance, as it is difficult to assess if the term is being used in a meaningful way. "Paradigm" and "synergy" are good examples of such powerful yet tricky concepts prone to misuse and buzzification.
Some philosophers say that the meaning of a word is determined purely by its usage. In this view, a word like synergy loses much of its meaning, as it is commonly used in such a jumbled and unclear manner.