Apparently it was possible to unlock Queen Sigma in previous versions of the game, where there was less things on the collection page. So if you already had her you got her weapon as well. It's just that as things were when I played the game it was necessary to have her weapon to achieve 131 unlockables. Might have been an oversight, and I'm not sure if it was fixed. What I did though is that there is a code to unlock Queen Sigma, and with that I was able to complete the collection.