It is true that there are organic acids, like tannic acid, found in oak leaves, and tea leaves. And it is true that these organic acids will lower soil pH, but not very much. If your soil has some clay content, and the clay contains limestone particles, the resulting soil will have pH above 7, and additions of composted leaves will have little effect on pH, maybe 1/10 pH unit. Even so, plants do well in clay soil amended with composted leaves, so that is generally a good idea. Agricultural sulfur, aka soil sulfur, is one way to slowly lower soil pH. What is the initial pH of your soil?

Much like coffee, if the leaves/grounds have been used to brew the drink, much of the acid has been removed. Used coffee grounds (UCG's) and tea leaves will have some tannins left in them but far less than if used before the brewing process. Laboratory tests sponsored by Starbucks have shown UCG's to be only very slightly acidic to almost neutral with regards to pH.

Either way, they are not an effective method of lowering soil pH - they just offer too little to make much of an impact. Same with most leaves and conifer needles -- contrary to common belief, tests have shown that these materials generally considered to be "acidic" amendments or mulching ingredients have minimal impact on soil pH. They may offer a slight lowering of pH at the soil surface if used as a mulch but once incorporated in to the soil and beginning the decomposition process, their contribution to soil pH is negligible. And once decomposed, like other compost, they will be nearly neutral in pH.

Using the brewed liquid rather than the remains should provide better results - that's where all the acidity is - but even that is not going to do much and not for long. Soil pH is dependent primarily on the mineral component of the soil, not on any sort of organic matter that is added to it. Organic matter decomposes and disappears over time while the minerals remain. Altering soil pH to any significant degree is an ongoing process, not a one time thing.

You'd have better results with applying sulfur or incorporating peat moss in the planting hole but these methods will need to be repeated as well.

All of the studies I have seen from our Agriculture Research stations show that adding organic matter, tea leaves, coffee grounds, Oak leaves, Pine needles, or teas made from them will not significantly affect soil pH. You need to change the number of free radical Hydrogen ions in your soil to do that, with sulfur.