Mulberry can be pruned, you do it midwinter so that the latex doesn't bleed. In places like Greece, where they are grown as street trees, they are routinely pollarded to keep them as a fixed size, typically about 8 feet or so. However I suspect you may lose the fruit if you do this, though I'm not sure. Perhaps the Greeks don't want the fruit dropping on the floor, they like the big leaves to create shape. I trim mine most years to keep it manageable, and the fruit within reach. I'm guessing that if you want to get much of yield of fruit off it, you'll need to let it get to a reasonable size, as it doesn't fruit with the density of a plum or blackcurrant. But I've not seen one grown as a bush or in a pot. But if the people who are recommending this are credible, and are talking about the same plant as you (there is some confusion over names), then perhaps they are right.