Is direct current really better for pumped-storage hydroelectricity facilities?
Perhaps it would be better to say "direct current is beneficial to hydroelectricity plants".

Wikipedia shows several hydroelectric power plants use HVDC to transmit power, including
Volga Hydroelectric Station,
Tianshengqiao-I Hydropower Station,
Itaipu Dam,
Cahora Bassa,
Inga Dam,
Tianshengqiao-I Hydropower Station,
the various hydro plants that feed into the Radisson Substation,
the various hydro plants that feed into the Nelson River Bipole,
Benmore Dam,
Sakuma Dam,
Three Gorges Dam,
etc.

Why would using direct current at a plant be beneficial?

Wikipedia: HVDC has a good explanation of why HVDC (sometimes) is better than AC. In summary:

With a given long-distance transmission line, HVDC has fewer losses than high-voltage AC. Both have roughly the same parasitic resistance losses, but only AC has losses due to parasitic inductance and parasitic capacitance.

With a given long-distance transmission line, HVDC can transmit more energy per minute. HVDC can transmit at maximum power all the time (the maximum voltage and maximum current supported by the line), while AC has zero crossings where a line is not transmitting any power.

HVDC is immune to some kinds of failures that occasionally hit AC systems, such as loss of synchronization.

The main disadvantage of HVDC is the cost of inverters to convert power to 60 Hz AC.
So these HVDC systems are point-to-point, with inverters only at the two ends, rather than a heavily branching structure which would require many inverters, one at each endpoint.
As the cost of inverters continues to drop, I expect HVDC to be used more often.

As far as I can tell, all these advantages would also apply to pumped-storage hydroelectricity facilities.