They are using Coulomb forces to produce fusion. That's what a Fansworth-Hirsch fusor does, as well as a line of related fusors like the Polywell which I'm guessing will never produce net energy. For this application net energy is not necessary, just enough additional heating to get enough additional ISP to increase the amount of delta-v per-kg of propellant -- the additional weight of the power plant and fusor must not make this so inefficient/expensive as to lose that ISP advantage.

I doubt they have any additional power plant or a fusor. They’re just injecting boronated water into the proton exhaust, and presumably getting enough fusion reactions to be useful.

“The fusion process was first devised during a R&D program for the US Air Force’s AFWERX (Air Force Work Project) initiative, where boronated water was introduced into the pulsed plasma thruster’s exhaust plume. This created alpha particles and gamma rays, clear indications of nuclear fusion.”

It sounds fairly crude, but if it works it works.