Setting target to "ES5" does not build to ECMAScript level 5

E.g. using the following npmextra.json:

{
  "mode":"custom",
  "test":true,
  "npmts":{
    "ts":{
      "./ts/*.ts":"./"
    },
    "tsOptions":{
      "target":"ES5", 
      "declaration":"true"
    }
  }
}

Still outputs js that uses ES6 class etc.