There is a HUGE difference between ultra-thin films of water that are able to slowly -- slowly as in geologic time -- weather the outside of a rocks, one tiny speck at a time, and having enough water present at any given time to produce something even close to what most people would call "mud".

Did you actually read the whole abstract? Yen, et. al., make it very clear they are talking about films of water a few molecules thick, which even then only last for a few minutes at a time, which enhance weathering over "geologic time scales" (their words).

It's clear that the floor of Gusev has pretty much been sitting there for 3 billion years (given the crater density). If that soil were getting wet enough to form mud we'd see a hell of a lot more weathering than slightly enhanced oxidization of iron minerals.