The fundamental is around 200kHz but the harmonics extend up to 100MHz or more (for typical regs; I don't know LM4164 specifically).
You have basically ~pF of capacitance from that node to chassis, pushing your circuit up by as much. If there's 100pF GND-chassis capacitance, that attenuates it up to 40dB maybe, but it's coming from 10s of V and you need ~mV. The LISN has ballpark 50 ohms to chassis (per line, and give or take how the impedance works out at the far end of a cable), so the equivalent circuit is a capacitive divider into a resistor load. The Thevenin equivalent source is ~100pF at 100s mV, into 50 ohms; below cutoff (Fc = 1 / (2 pi R C), ~32MHz), it's asymptotic 20dB/dec, and 200kHz is 2 decades below cutoff so you get another ~40dB there.
It should be close, actually. But it's all proportional to switch node amplitude and capacitance. Easily solved with a larger capacitor (some ~nF) from GND to chassis, and even moreso with a CMC or individual chokes inline. The CMC must be used with capacitance to chassis, not to circuit GND.
The spectrum will be ~flat up to some upper cutoff, because square wave harmonics go as 1/N, balancing the cap divider's response. In the span from ~30MHz to that cutoff, amplitude may be up or down -- how much depends on ringing at the switch node or other points, and finally the rise/fall time of the regulator.
Tim