Those are legit knockdowns, regardless of what they are proving or not. Nobody here said Chuvalo couldn't take a great punch, as far as I can tell.
Foreman was swinging and landing very few punches cleanly, the first left hook was probably the only one where Chuvalo was in the process of throwing his own punch - the punches that are most devastating, when you are throwing a hook/swing, but your opponents gets to your jaw first. Now compare that with several such punches which Foreman landed on Lyle before he knocked him down the first time. Foreman was the hardest puncher Chuvalo faced, and three or four clean punches were enough for him to be defenseless and not knowing where he was and what he had to do (punch back). Even Frazier was trying to punch back. Chuvalo was clueless.