Apologies, you're quite right, they're using "transcranial alternating current stimulation" a.k.a. tACS during sleep. Now I'm at my desktop I can read the paper, and see that they applied direct current during task training (discovery of "hidden targets in static images") using the "StarStim R32 simultaneous EEG/Stimulation device", and their own tACS system during sleep.

ECT(Electroconvulsive therapy) is already a thing - it is used for various treatments for depression and seizures. It consists of small electric currents passing through the brain which is very similar to brain stimulation, the only difference is that he first one is a medical procedure while the latter is, as of now, more like a brain hack. I wouldn't recommend anyone doing it without knowing the risks involved.

I don't think it's accurate to describe ECT as using small electric currents. The very name says that the procedure induces convulsions.

From Wikipedia: "ECT is often used with informed consent as a last line of intervention for major depressive disorder, mania, and catatonia. ECT machines have been placed in the Class II category (special controls) by the United States Food and Drug Administration (FDA) since 1976." (Emphasis mine.)

It sounds like hard rebooting a mind.

I feel like it sounds like a really bad idea.