I believe the k in “15k” is a suffix and thus not beholden to the rules of SI prefixes. In this case it is used as shorthand for “thousand”, and this usage is prevalent even in countries where SI units are not commonly used.

Insisting on changing 15k mAh to 15 Ah is the same as insisting on changing 15000 mAh to 15 Ah - which seems to be a matter of personal preference more than anything.