I'm guessing he removed the damping resistors. Those would normally kill the Q of a tuned circuit, but the resonance could still cause an issue. I suppose the resonance could be caused by a very long cable run from the antenna to the amp, but the low 15 Khz resonance point makes it look like there is an intentional tuning capacitor across the antennas.
I don't believe the manual warns about resonant antennas specifically, other than cautioning about self-resonance and using excessively long connecting wires/twisted wires between the amp and H-field antennas.
What is clear is this guy's station is contributing to, occasionally, over 80% of the detected Region 1 strikes, and degrading the accuracy in the process.
Don
WD9DMP