This particular scenario involves cornering while hydroplaning. I don't know what other people's experiences have been, but in my case at least 95% of the time I've experienced hydroplaning I've been going straight. The moments where I've been closest to losing control happened when hitting unexpectedly deep puddles on the highway. In those cases, if the front wheels remain planted but the rear wheels are loose, you still have directional control to keep the car straight. The car behaves like a weathervane, the rears follow the fronts and there's no oversteer and no big deal. But if the front wheels break loose, you have no directional control and that could become a big deal if there's any angle between the direction of the wheels and the direction of vehicle travel, which could result in a sideways force from the water pushing the car further offline.
The other wet weather scenario that I think is more important is braking. I suspect that failing to get the car slowed or stopped in time is a bigger cause of accidents than oversteer. For best braking performance, the best tires should be on the front. This is especially true in snow, which for those of us who live in the North is more of a concern than hydroplaning. Oversteer in the snow is (for me) relatively harmless and very easy to correct, understeer is worse, and traction while braking is of the highest importance.
Yeah. it's funny that Tirerack or Michelin didn't test for wet braking with best tires on rear vs front, or as you mentioned, driving straight at reasonably high speeds in wet weather with best tires on rear vs front.
Their conclusion lists one example, hauling ass around corners, which most people will not do anyway when it's raining, and this justifies their reasoning to them. It just also happens to create a situation where drivers will burn through tires faster and void any kind of mileage warranty if they replace only 2 tires at a time.