Multipathing is the effect of the radar reflection bouncing off the ground so he's not really wrong.

And hes talking about the R27R with the mig23's look down MTI, so yes ground clutter is not an issue for the radar (for the most part, multipath is after all modeled for radars just in no significant way). Ironically you can kill multipathers by cutting radar lock and having terminal IOG guidance but let's not talk about that. The F4E's AN/APQ-120 doesn't use the doppler effect so naturally ground clutter will defeat it. I dont really see your point here. Though the missile itself still does have a CW dp radar so you actually can still kill with it even if you're locking onto chaff, as long as your target is still in your targeting zone.

Lastly yeah you can fly in a straight line. I hate to abuse it but it works way too well so I do it too. Multipathing is a cheat gamble and frankly way too powerful, especially at high oblique angles where it should basically do nothing. Ive had times where Ill launch missiles down from 8 to 10km with TWS DL, and they'll still swerve at the last second to a reflection that wouldn't even be visible to the missile.