Well it seems like that not what they want.If they gave the recent update, a chunk of content, I am sure many would recommend this game to others as well. And they would get a boost in their patrons as well.
In the end it all comes down to "spend time / paid" ratio. From this point of view with 600 paying patrons, it might be just more profitable to do nothing for almost a year and then upload the update containing just two scenes (which can be considered as regular monthly update in some other games).