Forget about postUpdate.
You don’t need it. You are not using it. It is irrelevant.
Everything is a command.
To send a command from Habpanel you must use a controlling widget. Dummy is text only. You cannot cause anything to happen using a Dummy widget. You probably want to use a Button or a Switch and use a Switch Item.
Now to address your misconception.
ANY state can be sent using postUpdate to ANY Item type. The difference is one uses postUpdate to change the state of an Item internally to OH where sendCommand activates the binding/channels linked to that Item. So, for example:
MyLight.sendCommand(ON) // goes to the binding and causes the light to turn on
MyLight.postUpdate(ON) // changes the state of the Item inside OH but the command does not get sent out to the device
That is the only difference. The String issue you read about is that the postUpdate Action only can take a String as the state. This String gets parsed into whatever is appropriate for that Item type. So, to postUpdate a numerical value to a Number Item using the postUpdate Action you need to do:
but if you use the postUpdate method you can pass the actual number and avoid the parsing: