There's no particular reason to believe they are Muslim refugees - they could also be Christian - while Muslims are the largest grouping of refugees into Australia, about 30% are Christian. People seem to be just assuming that the ones under consideration are Muslim - they could be, but to be honest, I suspect that as Australia does want them settled in the US, it's quite likely a lot of them are from the Christian groupings.
What I mean by that is Australia does want the US to accept them, and as the US is more likely to accept Christians as being less potentially dangerous, I can see those presented being from that grouping. Australia’s officially ‘blind to religion’ refugee policy means that it wouldn’t be outlined in those terms publically, hence some of the secrecy behind the deal.
*** There’s no particular reason to believe they are Muslim refugees ***
Except for the news reports out of Australia.. The 870 male only camp in Papua New Guinea, and the other camp 9n Nauru hold mainly Muslims. A list of the countries they hail from are Muslim countries.