If, as NYSRPA suggests, you have to stop looking at restrictions as of 1868, then the decision is correct. There were no restrictions on non-citizens carrying nor any federal laws restricting immigration until later.
This. We can't have it both ways. If "text and history" is what we're hanging our hat on for RKBA, then we have to take every part of that text and history into consideration.
I'm not a fan of illegal immigration, but I'm even less a fan of government infringing on RKBA. So this ruling is a win. It weakens the government and allows more people to defend themselves; any downside, I think, pales by comparison to those two goals, which are (if I need to remind anyone) the main reasons why 2A was written.