Actually I’d say that makes the least sense. There are a plethora of other Mobil 1 full synthetic oils that do not say FS on the label. Of course there are lots of others that are also full-SAPS that don’t say FS, but all of them say full synthetic on the label. As has been noted ESP doesn’t say FS.
So yeah I’m in the camp that it means full-SAPS. I mean why be redundant with a little obscure “FS” on the label to indicate your strongest marketing attribute of being a full synthetic oil? They makes no sense at all.