Like most good rumors, there is some kernel of truth. There are some limited situations where a DC-DC charger could be warranted, but in most applications they are not needed. They are certainly not needed to prevent 'surges'.
Sterling and Battleborn are in the business of selling things, if they can convince/scare you into buying more stuff from them, better for them.
Battleborn in particular has some shady marketing practices, so I am not at all surprised that they would try to convince you to buy a $350 DC-DC charger (that they conveniently happen to sell) to 'protect' the $1000 battery you just bought from them.
Kind of reminds me of this:
Fargo - TruCoat
