Using acidic water is important for digestion, skin mucus (actually removing the part that is not required), skin itself (as it is uncomfortable for the fish to have the pH set to high) and so on..
Of course there are some bacteria and fungi in this water. There is a stream, somewhere in Europe, where pH drops as low as 2! And there is still presence of some lifeforms.
The differnce between Bornean swamp and British soil is the insanely thick layer of peat over the aquifer. In such a situation there are almost no bacteria or fungi that are able to travel through the peat barrier, as opposite to the soil, where the transfer of mikrobes is easy becouse of almost stable pH throughout the whole volume of the soil. In case of the peat-barrier the pH changes drasticly from neutral to very acidic, so many bacteria and fungi are killed in the process.
At least this is what I read and got from my deduction.