A lot of the fine particulates are mobile globally. It doesn't take much in the way of similar processes plus global mixing to produce apparently identical results.

The case for a global ocean is not met until the problems it would bring are solved - in esscence, there is no erosion record of a global ocean. No sea cliffs, no beaches, no river deltas*, no shorelines (Despite Tim Parker's earnest efforts).

The Sulphates at Meridiani can't have been produced by an acidic ocean - the topography is all wrong. if there was an ocean there, it would have been open to the whole northern plains, and there would there would be similar sulphates at other sites around the periphery of the ocean. Instead, Meridiani is essentially unique as a widespread sulphate area.

Other sulphate areas are formed as mounds within craters and in the major "rift valleys" of mars.

I much prefer a hydrothermal explanation of the sulphates - the mounds are essentially giant tufa mounds where crater or rift hydrothermal systems have surfaced, and Meridiani is an area of surface seeps and encrustations, not an open ocean. But that's just my view...

* Well, no global pattern of them. There are a few isolated deltas in some craters, implying that at some time, there were crater lakes, at least locally.