Yep, definitely tons of layered guitars. I just finished a song that has similar guitar work to the Pumpkins style of riffery. I used 4 tracks of guitar panned slightly different with different eq's and settings. Very much a Pumpkin like tone from Siamese. I played all four tracks separately, and this definitely nails the little differences in playing, but sums together nicely.
This leads me to think it really was an army of guitars layered together using different octaves. This is such a part of their sound. Try it and see what happens. I bet you get a much better tone doing this, than anything a harmonizer can give you.
It may be more work this way, but it's a cool way to layer tracks and get a vibe going. Siamese is still one of my favorite records to this day. I wish they had done it with a bit less compression, but it still sounds nice. Good luck