At best, this is an argument not to use non-exponential, translation invariant discounting.

It is an argument not to use non-exponential, discounting.

You can discount in a way that depends on time [...]

Exponential discounting depends on time. It is exponential temporal discounting being discussed. So: values being scaled by ke^-ct - where the t is for "time".

The prevailing interest rate is normally not much of a factor - since money is only instrumentally valuable.

or you can not discount at all.

That is the trivial kind of exponential discounting, where the exponent is zero.

I know of no such justification for exponential discounting. The wikipedia article does not provide one.

The bit I quoted was a justification. Exponential discounting yields time-consistent preferences. Only exponential discounting does that.