Some (noris digital pencils and older wacoms) don’t have a button but an eraser instead so a binding would be better (with default binding=pencil button). multi touch works quite ok though.
Ok - figured it out. You have to enable sculpt via finger (in Gestures - I disabled it to Stylus only, to improve the overall sculpt experience).
You might also consider to add a hint in the pop-up menu? Like: “Single tap with second finger to switch mode. Android: non-Stylus only.” For the other Android Noops like me.