These words are confusing, because well decomposed manure is effectively compost and I do say that on page 14, thereafter using the word ‘compost’ to describe any well decomposed organic matter, of whatever origin.
I think that confusion arises because some writers use the word manure for almost-fresh excrement, which is not good for roots and can burn them, but all manure of at least six months age, and which has been stacked with enough air in the heap to stop it putrefying (anaerobic decomposition, bad smell) is fine to use as surface mulch and then to plant through.
How much your plants need is hard to say and if there are few weeds, or only annual weeds, a three inch layer with no cardboard may do it. And your 2-3 year old manure sounds great for growing. As for using too much, yes you could if you applied 6 inches per year and did not grow intensively. But the "French intensive" market gardeners around Paris and other big cities, in Victorian times for example, used to apply over 100 tons per acre, and grow three crops a year or more. I reckon that the right amount is whatever will mostly disappear within a year, devoured by soil organisms.