Maybe coffee grounds got their reputation for acidity when someone piled them up deep, or made a compost heap with lots of coffee grounds.
Those fine grains clog up any air spaces, tending to make a layer or heap anaerobic. That causes fermentation to replace respiration.
That causes the incompletely oxidized fermentation products to be released, and most of those are acidic (or a little toxic, both). In fact, many fermentation products ARE organic acids, like acetic acid / vinegar.
However, the grounds themselves have very little acid after you brew with them! As long as you don't make your compost heap or sheet mulch anaerobic, they should contribute little or no acidity.
10 kilograms per square meter is the same as one gram per square cm. If that is "wet weight", I think the layer would be less than 1 cm deep (less than 0.4 inches). If it was dry weight, I'm guessing it would be around 1 inch deep.
Is that deep enough to go anaerobic? If it is, they should have been raked into the top inch or so of soil, or mixed with coarser mulch.
