Brains of different people are not in a one to one correspondence, they do not have the same number of cells and even if they had, it is not known if the same information will get encoded in the exact same cell.
This is true, of course, but irrelevant to this work. At the level they're working at (fMRI scans, which have a resolution on the order of 0.5-4mm or so depending on the temporal resolution, etc.), you can't resolve individual cells anyways so you don't have to be concerned about those kinds of individual variations.
Visual activity in many parts of the brain follows a retinotopic map, where activity in nearby locations on the retina are processed in nearby regions of the brain. So, while you would have to calibrate some details, a lot of things would be constant between brains.