Is disk brake effectiveness mitigated by tyres losing traction under strong braking?
Ideally, the brakes should be able strong enough to break traction otherwise you will not be able to brake maximally (i.e., just before the threshold of losing traction). Rim brakes can do this, even in wet weather, if set up correctly.  The real advantage of disc brakes in wet weather is other aspects of how the brakes perform.

From personal experience, disc brakes have superior wet weather performance for the following reasons:


Modulation: as stated above maximal braking is when you are on the threshold of losing traction.  Good hydraulic disc brakes are typically renowned for their feel and modulation (i.e., enacting finite changes in brake force), letting you approach those thresholds with more control.
Response time: while a well set up rim brakes can generate strong braking forces, the response is often delayed in the wet due to the need to first clear the rim of water before effective braking can occur. In a similar manner disc brakes must clear water from the rotor, but this tends to occur much more quickly due to rotors being better protected than rims and the slots in the rotors giving water a place to go when the bake pads are engaged. This gives disc brakes responsive brakes even in down pour conditions.
Wear/longevity: rim and disc brakes both use some type of brake pad, and in both cases these brake pads will wear faster in the wet. That said, brake pads associated with rim brakes tend to wear proportionately faster due the rim braking surface being closer to the road causing it to get dirtier quicker than a disc brake rotor which is close to the wheel hub. 
Structural integrity: Disc brakes have a dedicated braking surface (i.e. the disc rotor), where as with rim brakes the braking surface is built into the rim.  Wearing the braking surface of rim brakes can eventually result in rim failure without regular inspections and replacements. Where as rim brakes require a wheel replacement or rebuild, with disc brakes wheels should theoretically last longer as it is fairly trivial to replace a disc rotor. Furthermore, damage to a rim will affect braking for rim brakes, but not disc brakes.


In dry weather and/or clean conditions there is arguably less of an advantage to disc brakes compared to a well set up rim brake.