Also speculating, but here goes.
Vmc is the speed at which you no longer have the control authority to handle the assymetric thrust caused by a failed engine.
If we assume that this is not for maximum/takeoff power, but rather for a power setting which will get you out of the situation intact without exchanging altitude for speed (i e enable you to accelerate in level flight), then we are on to something.
In ground effect, you will need less power to accelerate out of the low-speed region. Less power in the engine out scenarion means less torque caused by thrust assymetry and a lower Vmc since you’ll need less airflow over your control surfaces to counter the assymetric yaw.
Note that I’m talking actual Vmc here, not the FAA approved one...
Cheers,
Fred