The default was whatever the provider of the programming software decided, at that time VEX only worked with partners to provide programming solutions. RobotC, EasyC and later Robotmesh all supplied software, none (IIRC) were free.
The Purdue team revere engineered the cortex and created PROS somewhere around 2012/2013 (as did I, not working for VEX at that time convex), it was targeted at competition teams which is probably why it was expected to be used with VEXnet keys and a controller.
RobotC had a setting for standalone but not saved with the project, same with EasyC I think.
I believe standalone mode allows the cortex to operate without either a USB connection to the PC/controller or a VEXnet key (it’s amazing how quickly we forget this type of thing despite working with the cortex for several years), it’s nothing really to do with autonomous or driver control modes which, again, are under control of the programming solution which would have code monitoring the competition state.