JSON в многомерный массив

Вам следует прекратить использование SwiftyJSON и перейти на Swift 4 и Decodable:

struct User : Decodable {
    let date : String
    let username : String
    let submitid : Int
    let notes : String
}
struct Result : Decodable {
    let data : [[User]]
    struct AnyCodingKey : CodingKey {
        var stringValue: String
        var intValue: Int?
        init(_ codingKey: CodingKey) {
            self.stringValue = codingKey.stringValue
            self.intValue = codingKey.intValue
        }
        init(stringValue: String) {
            self.stringValue = stringValue
            self.intValue = nil
        }
        init(intValue: Int) {
            self.stringValue = String(intValue)
            self.intValue = intValue
        }
    }
    init(from decoder: Decoder) throws {
        let con = try! decoder.container(keyedBy: AnyCodingKey.self)
        let intermediate = try! con.decode([String:[User]].self, 
            forKey: AnyCodingKey(stringValue:"data"))
        var data = [[User]]()
        for d in intermediate {
            data.append(d.value)
        }
        self.data = data
    }
}
// jsondata is your original JSON data, as you downloaded it
let result = try! JSONDecoder().decode(Result.self, from: jsondata)

После этого result.data представляет собой массив из массива User.

[[User(date: "2018-07-03", username: "user3", 
       submitid: 15302930963080, notes: "Hello")], 
 [User(date: "2018-06-27", username: "user1", 
       submitid: 15302992338145, notes: "Testing"), 
  User(date: "2018-06-28", username: "user1", 
       submitid: 15302992338145, notes: "Testing")], 
 [User(date: "2018-06-21", username: "user2", 
       submitid: 15301374235890, notes: "Comments one two three"), 
  User(date: "2018-06-22", username: "user2", 
       submitid: 15301374235890, notes: "N/A")]]