I tested with this filter:

{
  "presence": {
    "limit": 0,
    "types": [
      ""
    ]
  },
  "account_data": {
    "limit": 1,
    "not_types": [
      ""
    ]
  },
  "room": {
    "rooms": [
      "!a:b"
    ]
  }
}

I expected a very small response, particularly because I filter almost everything out. However, I end up getting a full sync response (with all my rooms). I also shouldn't have to supply the types or not_types (those are me trying to figure out why the limit doesn't work), and rooms: [] should surely prevent all rooms.