"The AUTO BRAKE DISARM indicator light illuminates when the
automatic braking system is turned on to the RTO position. The
light will remain on until a turn-on self-check (TOSC) initiated by
the system is successfully completed in about one second. The AUTO
BRAKE DISARM light also illuminates for both failure detection
(including power loss) and crew-initiated disarming."
From the Maintenance manual. In the aircraft, this is what happens regardless of the hydraulic pump switch positions. If the light stayed on with elec hyd pumps off (as some airlines set up), there would be no proof of this self-test. As you say, sounds like a sim glitch.