I solved it -
1. I display the polygons via a graphic layer whose elevationInfo property you'll set to "on-the-ground".
2. I use SketchViewModel whose layer property is that graphic layer, and whose defaultUpdateOptions property is:
defaultUpdateOptions: {
tool: "reshape",
enableRotation: false,
enableMoveAllGraphics: false,
enableScaling: false,
reshapeOptions: {
shapeOperation: "none"
},
toggleToolOnClick: false
}