Richard doesn't know how the Hertz clock was powered. He just makes this crap up as he goes along.
I can't say with certainty but this passage from A&C appears to indicate the sign was electronic rather than battery powered and was turned off to save energy (again suggesting that is was powered with electricity). But rather than just being your usual contrarian ...... self, why don't you contribute something of substance to the discussion if you know differently?
"The Hertz Corporation expressed no interest in its abandoned advertisement, having permanently turned off its
electronic time and temperature function on December 11, 1973,
during the United States energy crisis." Assassination and Commemoration - Stephen Fagin - p.70