Anyone claiming to have flown both the '21 and the F4 and claiming they'd prefer to fight even in a late model F4 is either a liar or a fool. Or an overly-optimistic American.
John Nash (1st topgun instructor) had said that he'd have loved to take the Mig-21 to war put a tailhook on and use it in combat. But effectively it was a sports car not a great fighting machine. And he was the person evaluating it and he loved but....
a Red Eagle (few years after Smash Nash) , Paco Geisler, with 500 Mig-21 sorties (almost wall-to-wall ACM) loved the plane, defeated all teen jets with it and went out stating: "I wouldn't want myself anywhere near that thing in combat!"
You draw you own conclusion.*
The Phantom was vastly superior. OK not vastly but decisively. The Israelis in fact took the F-4 1v1 in a low speed rolling scissors, more than once against the Mig-21 defeated it...in fact they killed the Mig-21 by just flying it into the ground in an environment where the latter was supposed to be significantly superior...or not quite? Anyway, you get the point....
To go 1v1 slow against the 21, well I'd say they were either inept or supremely confident in both their abilities and in knowing/quickly assessing their enemy...
*The Lightning was in many respects a great MiG-21, but still a MiG-21. You'd rather go to war in the F-4. As much as I love the Lightning over and above the other 2 a/c...