Testing a big more, just dropping "dlig" from the feature settings declaration turns off the awful st ligature. So they don't have to 'all' be turned off to get rid of that one. What I don't know is what other ones get turned off by dropping "dlig" from the declaration.
font-feature-settings: "liga", "dlig";
So just remove that clause from your stylesheet and you'll be rid of that ligature.