This was an upgraded aircraft and moreover even the tube radar altimeter would give some indication of reduced altitude. You’re not in a fixed wing aircraft where forward momentum is the only way to change direction/speed.Radar altimeters are not precise at low altitudes, let alone a tube one from fifties.
Very likely they just turned off the warning.
This is a key distinction in between a forward looking terrain avoidance radar, vs simple radar altimeter. The prior does a lot of computing to figure out a dangerous flight envelope.
So, they put aftermarket upgrades on their helos. Then this turns much less clear.