I don't think so. In analysing designs for perpetual potion machines you're allowed to assume perfect efficiency, unless you can show it to be physically impossible. Here we only need to assume the conversion is sufficiently efficient. That's within the laws of thermodynamics, and the machine still works.
The problem lies elsewhere.
EDIT: In response to a down-vote, let me explain further.
By the laws of thermodynamics, even a perfectly efficient machine cannot result in a net production of energy. In the analysis of an engine the assumption of perfect efficiency is allowed, because really one is saying "assume sufficient efficiency that the production of energy will compensate."
In the case here we're assuming that we can convert mass to energy and vice versa and getting net production of energy. That shouldn't be able to happen, and the falw isn't in the assumption of perfect efficiency. The flaw is elsewhere, and the challenge is to find it.
I do know where it is, but it took me a long time to find it. I'm sure many people here will find it much more quickly than I did.
But the flaw is not in the assumption of perfect efficiency in the mass/energy conversion(s).
you're allowed to assume perfect efficiency, unless you can show it to be physically impossible
Even then, his specification is at too high a level of abstraction for us to prove that it's physically impossible. That seems unfair.
If the author wants people to ignore the abstractions that quickly prove something impossible, then he or she should provide enough detail to analyze it from a practical perspective.
The problem lies elsewhere.
EDIT: In response to a down-vote, let me explain further.
By the laws of thermodynamics, even a perfectly efficient machine cannot result in a net production of energy. In the analysis of an engine the assumption of perfect efficiency is allowed, because really one is saying "assume sufficient efficiency that the production of energy will compensate."
In the case here we're assuming that we can convert mass to energy and vice versa and getting net production of energy. That shouldn't be able to happen, and the falw isn't in the assumption of perfect efficiency. The flaw is elsewhere, and the challenge is to find it.
I do know where it is, but it took me a long time to find it. I'm sure many people here will find it much more quickly than I did.
But the flaw is not in the assumption of perfect efficiency in the mass/energy conversion(s).