One interpretation of "high frequency" is the formal HF frequency range of 3 to 30 MHz. Same sort of possibly-formal usage as VLF, VHF, UHF, etc. Seems unlikely in this case, but I'll be generous.
About 25 or 30 years ago, I read an article where the author questioned the application of Carl's diode as "protection" in the output power amplifier final stage of low power HF transmitters. His analysis indicated that the relatively high junction capacitance of the zener diode had a larger effect than the Zener or avalanch IV curve.