Well correct and true for the jets I have flown with the caveat that it takes a darn slick runway to get a high speed loss of directional control going based on reverse thrust application. But yes, if that occurs you decrease reverse thrust to regain controllability.
For example, an MEL'd TR would be allowable for many jets with a wet runway braking action good report. Also, you can honk in a full helping of reverse on a single engine landing and have zero directional control issues with wet-good runway on evey jet I've flown. ( but I've not flown the mad dog)
I think what we are talking about here is both. I think the rudder blockage issue sounds like its a serious factor on the MD and that runway was slipppery as snot, probably more than the breaking action report provided.