Update: turns out both my battery and the MyVolts failed (probably unrelated but who knows, battery was old). The MyVolts folks sent out a replacement for the Digitakt 12V cable, and it works just fine with my new battery. They also sent me a reply clarifying that the statement applies to sustained usage and it should deal with the described conditions just fine. I don’t know whether or not those conditions caused my first cable to die, so I won’t speculate on that.
I’m also currently using their 5V cable designed for the Model:Cycles with success. That’s much more of a no-brainer, since there’s no voltage step-up involved… just a format change from USB to barrel.