The TAWS is ACSS T2CAS according to Sukhoi's webpage here:
Sukhoi Company (JSC) - Airplanes - Civil aviation - Sukhoi Superjet 100
This same TAWS is optionally installed in Airbus A320 (instead of Honeywell EGPWS), so it's not surprising the TAWS control panel looks identical. You can see an interactive Airbus TAWS control panel here:
A320 Ground Proximity Warning System (GPWS) & Cockpit Voice Recorder (RCDR)
The TAWS "TERR" button is a locking switch/annunciator. It displays FAULT when the TAWS functions are not working (due to lack of power, or IRU, or position, etc) and when the switch is depressed it will display OFF to indicate the TAWS functions have been manually inhibited. I don't think OFF illuminates for faults, otherwise the crew would not know the position of the terrain inhibit switch (active, deactive).
If they were flying low to give the passengers a thrill, it is likely they would have activated the terrain inhibit.
This ACSS T2CAS has a unique feature that voices "Avoid Terrain" instead of "Terrain Terrain Pull Up" if it believes (based on the internal terrain database) that the aircraft might not be able to out-climb the terrain. In that case, the crew is supposed to turn the aircraft instead of performing max climb.
Of course, which way to turn will be based on the terrain display so hopefully the terrain database and the position of the aircraft are correct, otherwise the pilot could be mis-led into turning toward higher terrain instead of away from it!!