He's probably going to go with the "we are all brothers in Christ" thing. But it is very possible he misses that detail. Joshua is very lightly flavored Mormon with it mattering because he came from Utah and having the 1911 (invented by a famous Mormon gunsmith) be his "tribe's" signature weapon. IIRC he quotes the old testament a lot and some new testament stuff too. But I don't think he or the other missionary ever refer to the book of mormon at all, but it's been a hot minute since I played New Vegas.