As I understand it, Spektrum have three versions of SRXL. None of which is the same as Multiplex SRXL. In all cases, the payload is similar to Spektrum remote receiver.
1) AR9020, nonstandard checksum.
2) AR7700, they got the checksum right.
3) 'T' receivers, for some reason, they decided to reverse the two bytes of the checksum.
Steve