Basically, they go something like SO42– → SO32– →S2O32–, which is broadly endergonic, but there's a ton of different electron acceptors in the vicinity which can essentially create the free energy, and also cycle the sulfur between these different forms. it doesn't get eaten up, just kinda tossed around.

It's not fully worked out the exact specifics of the metabolism, but it's some really weird anoxic stuff! the carbon deprivation is another big thing I don't even fully understand - apparently small amounts of carbon 14 are found in samples but it seems much of that is just because the samples are taken from surface outflow (same with how theres a bit of oxygen there, but almost no actual dissolved oxygen in the deep glacial reservoir)