I have noticed something else strange in the three waveforms in Reply #5. The first one, composit video and the second one, luminance (no chroma) show the 600 kHz signal increasing with higher video levels and almost completely absent at sync tip.
But the third waveform, the chroma one, which also seems to be displayed at the horizontal rate and locked to horizontal sync (note below), shows the 600 kHz signal all the way across the display. There is no place where it goes nearly to zero at the tip of sync.
It is not there (at sync tip) in the composit signal, which is the combination of luminance and chroma but it is there in the chroma signal. That does not make sense. If it is present in the chroma and not in the luminance, then the sum of those two should show it present. You can't cancel the signal present in the one with the lack of signal in the other.
Accurate DC levels are very important to the luma component of an analog TV signal.
If such a signal passes through an ac coupled amplifier or network, it loses the reference DC levels like black level, blanking, & sync tips.
These need to be restored by a process called "DC restoration".
Historically, this was done in various ways, but over time, the other methods have fallen into disuse, leaving the almost universal use of "keyed clampers" which produce a keying pulse synched to the line syncs, & either delay it a bit over 5us to be coincident with the "back porch", (the most common method) or somewhat less so it still falls within the sync pulse duration.
The keying pulses will cause the keyed clampers to hard switch either of those two video levels to a selected DC reference value during the duration of the clamp pulse, & would normally remove
any signals riding on that DC level, however, keyed clampers used on the "back porch" of composite colour signals incorporate "traps" parallel resonant at the colour subcarrier (soft clamping), so as to not "mangle" the colour burst.
Sync tip clamping, which, for a standard 1V p-p video signal, clamps the sync tip to a DC level of around -300mv w.r.t zero volts (exactly that for PAL) does not require such traps,
Perhaps Sony decided on the use of sync tip clamping to reduce the complexity of having to provide LC "subcarrier traps", & further decided to clamp the luma component both before & after it was combined with the chroma signal.
The Chroma signal doesn't have a DC level to be clamped, so the interfering signal isn't "clamped out".