The point of the article isn't just the voltage. It's that they created a photocathode which converts laser pulses into electron bunches with a particular set

The point of the article isn't just the voltage. It's that they created a photocathode which converts laser pulses into electron bunches with a particular set of qualities(polarity, density). It's also how they diverged from traditional designs and placed the HV supply outside the chamber(it is difficult to get 350kV through a metal vacuum chamber wall for somewhat obvious reasons).