Dorr is interested in how nominalists can account for the necessity of the claim:
(D) Whenever x is an electron, and y is a duplicate of x (or y is qualitively indiscernible from x, if you think that being a electron is non-intrinsic) then y is an electron.
He first discusses what he calls the physical strategy. The idea is that a science adequate to completely describe the actual world will provide us with the resources to give a real definition of 'duplicate':
(P) For x to be a duplicate of y is for it to be the case that x is an electron iff y is an electron, and x is a quark iff y is a quark, etc...
Unsurprisingly, this suggestion is deemed implausible, since it ignores the possibility of things being duplicates in virtue of sharing alien properties (nominalistically construed, obviously).
What surprised me was that Dorr suggested that our very own Joseph Melia might be a proponent of the physical strategy. This isn't how I understand Joseph's position, and is fairly straightforwardly and obviously inconsistent with the appeals to the possibility of alien properties that he and Divers make in the Analytic Limits paper, etc, so I was interested in why Dorr might have come to think this. I guess he must be going on the remarks on p 8 of Joseph's Truthmaking paper, where he talks about fundamental science being incomplete. Before that he has given solely actual world examples ('charged', 'square', 'mass'), but then, it is pretty hard to give examples of particular alien properties. And he seems to be restricting discussion to moderately sparse properties. But I don't see anything to motivate the idea that there is an implicit restriction to actually instantiated properties. On the contrary, Joe seems often to be pressing the idea that the sensible nominalist should be happy if he can establish that we can see how a certain sentence might be made true, or how the world could be a certain way in virtue of certain of its more fundamental aspects (e.g. why "it could have been the case that there were sets of duplicate objects that were ways that no actual things are" is true) without there being universals, even though we can't actually give a nominalistic paraphrase of the sentence. (This is something that Dorr himself seems to be happy with - he only requires 'paraphrase in principle', though one wonders whether even that may be too strong). That idea obviously carries over neatly to the aliens case. Once we see what is the truth of the sentences about non-alien duplicates consist in, it's not difficult to get a sense of what is explaining the truth of sentences about alien duplicates. And nothing which is illegitimate by the nominalist's lights seems to need brought in.
If it is the remarks on page 8 that is motivating Dorr's characterisation, I think that's a misreading. All Joe seems to be doing there is motivating the idea that we can recognise good truthmaker explanations even in the absence of paraphrase. That's something that ought to carry over very straightforwardly to the aliens case. Dorr notes that there have been 'few explicit defences' of the physical strategy, but suggests despite this, it is quite popular. I think it might be at least a little less popular than he suggests.