Unfortunately, it looks like the Sketch API only supports what the active page is.
https://github.com/sketch-hq/SketchAPI/blob/develop/Source/dom/models/Document.js#L547
Page.fromNative(this._object.currentPage())
Looking at the headers I was able to figure this out.
let sketch = require('sketch')
let doc = sketch.getSelectedDocument()
let nsarray = doc.sketchObject.documentWindowController().mainSplitViewController().sidebarController().pageListDataController().dataSource().selectedPages()
let jspages = []
for (let i = 0; i<nsarray.count();i++) {
jspages.push(sketch.fromNative(nsarray[i]))
}
console.log(jspages)
There's probably a better way to get at the pageList datasource but I didn't spend that much time looking into it.