There is a CASA AD from 1/2001 (AD/B737/149) for the B737-300/400/500, with regard to the autothrottle computer.
Boeing Alert Service Bulletin 737-22A1130, dated 24 Sept 1998 advises that replacement of the existing autothrottle computer with a new, improved autothrottle computer was necessary, due to the earlier autothrottle computer proving to be unreliable.
Part numbers are provided in the AD for the earlier model of computer, and Boeing advised that "Autothrottle computers with Part Numbers 10-62017-1, -2, -3, -4, -5, -11, -21, -23, -25, or -27,
may not be fitted to any aeroplane to replace an installed computer." (my bold and italics)
The background to the necessary autothrottle computer replacement was as follows;
"The aircraft manufacturer has received several reports of asymmetric thrust conditions during flight caused by irregular autothrottle operation in which the thrust levers slowly move apart, causing the aeroplane to bank excessively, and go into a roll.
This Directive requires replacement of the existing autothrottle computer with a new, improved autothrottle computer.
This action is intended to prevent a severe asymmetric thrust condition developing during flight, which could result in loss of control of the aeroplane." (my bold and italics, again).
https://www.legislation.gov.au/Detai...9-c9c2f7477bc3
IF .. and this is entirely speculative ..
Sriwijaya, in its regular penny-pinching mode (seeing as it specialises in buying only used aircraft, and that penny-pinching will almost certainly extend into maintenance - particularly under cashflow-affected COVID-19 operating conditions), fitted this aircraft with a "good, used" autothrottle computer, with the above part numbers - either in ignorance, or in disobediance of Boeings directive - it could be possible that the faulty, earlier version of autothrottle computer fitted, could have created a severe asymmetric thrust condition, which the crew were unable to diagnose accurately, and deal with quickly - thus causing the loss of control of the aircraft? Stranger things have happened.