Boeing told the FAA that MCAS was just an extension of the speed trim system.
1) MCAS was much more that a STS extension as the they were fundamentally modifying the logic of the stabiliser trim to accomodate it.
From the excellent and detailed blog by Peter Lemme at
https://www.satcom.guru/2019/08/conn...to-action.html
" 737 MAX Autopilot MCAS Trim Commands
MCAS ENGAGE is an FCC output discrete entirely unique to the MAX. It drives the COLUMN CUTOUT OVERRIDE relay in the column switch module. The relay serves two functions: to command high trim rate and to allow for AND command in the presence of AFT column cutout.
The MCAS ENGAGE discrete is routed to the actuator instead of FLAPS DN discrete to force high trim rate.
The AND trim command bypasses the AFT column cutout switch."
2) In the original MAX certification submission, Boeing listed the changes to the stab cutout switches as “Stab Trim cutout switches panel nomenclature” It was not just nomeclature, they were removing the ability to isolate MCAS/STS/autopilot inputs but still leave the Elec trim active. Again, why they would want to do this when they were going out of their way to minimise other cockpit changes has never been fully explained.
It will be up to the court to decide if these and other obfuscations were part of a deliberate attempt by the company to mislead the FAA regarding the impact of MCAS on other aircraft systems.