Is this a load of crap or sound science?
There are a few "sound science" cases where lower voltage would eliminate some waste. But probably not enough to pay for an "optimiser" anytime soon. And the "optimiser", itself, might turn out to be another energy waster.
But for PCs and other electronics with switching power supplies, it would make zero difference.
Electronics with linear DC supplies would benefit, but the question of the efficiency of the "optimiser" remains.
You need to look a little closer at what technology they're using, and at what independent
labs (not ones owned by/associated with the promoters) have to say.