Plant available Nutrients
There is a bit of confusion here about couple of concepts. So, let me address 1st nutrient availability and then acidity part of your question.
Plants can only take up ions from soil solution. Ions that are on the exchange complex (ie electrostatically adsorbed on negatively charged soil particles) need to be replaced (or exchanged through reversible reactions of ion exchange) with other ions from the soil solution. Through that exchange ions that were on the exchange complex are pushed into the soil solution, where they are available to plants. While ions are on the exchange complex, they are NOT available to plants.
Active and exchangeable acidity only refer to Al and H ions, not nutrients. But from where (soil solution vs. exchange complex) Al and H ions are available to plants, the explanation is exactly the same as described above.