The stock solution is just for the mix. Because both calcium nitrate and epsom salt are each a single nutrient double salt, there's no need to make stock solutions with them.
The discussion of mixing the nutrients is to highlight the necessity of mixing calcium nitrate into solution separately from the rest. This minimises the risk of "flocculation", a chemistry term for what happens when materials of high affinity interact in solution. I'm the case of calcium nitrate and epsom salt, if they interact at too high a concentration the calcium and sulfur bind into calcium sulfate, aka gypsum, aka wallboard, and fall right out of solution as a tan colored silt that your plants CANNOT USE hydroponically. So don't do that!
