As long as the prop can provide the same torque to turn the engine at the same speed, the supercharger will rotate at the same speed, and if the throttle hasn't been changed, the boost will be the same. N'est pas?
I don't speak french.
The propeller doesn't provide torque to the engine. The engine provides torque to the propeller.
So long as as the engine continues to rotate at the same speed, the supercharger will continue to rotate at the same speed, as I previously stated. This does not mean that manifold pressure will remain constant, even if the throttle position remains untouched...because manifold pressure is not merely a function of throttle position and engine RPM.
At high boosted settings, such as takeoff power, without touching the throttle position, during a power loss one will see a manifold pressure loss. The supercharger, despite turning the same RPM (assuming airspeed is adequate to drive the engine via the propeller to maintain that RPM) and despite the throttle position remaining unchanged, will show a loss in manifold pressure (trending back toward barometric).