Please read again. There aren't really any facts on this page that demonstrate whether or not LoP is an effective undergraduate physics textbook.
The page addresses two points: Feynman's fluctuating opinion of his own work, and the drop-off in student attendance (Feynman's opinion is then regurgitated as the third and final point). In fact, Feynman's opinion has no bearing on whether or not LoP is an empirically effective textbook. And regarding student attendance, the page argues against one person's recollection of dwindling attendance with another person's recollection (years after the fact) of steady attendance. Again, no evidence, just two contradictory and possibly flawed memories that have no bearing on whether or not LoP is effective.
The page provides as evidence an undated photograph of Feynman in front of a full classroom. When was this photo taken? Was it on the first day of class, or in the middle of the semester? Was it during a regular class, or a special lecture? We don't know.
Perhaps the LoP approach really is an effective textbook for teaching undergrads (despite Caltech abandoning it, no other nobody else to my knowledge successfully adopting its methodology), but the cited page doesn't provide relevant facts regarding the matter.
But the claims that it sucks for undergrads are also folklore, as far as I've seen. I wouldn't privilege them.
I took Caltech's intro physics sequence in the mid-80s -- IIRC my freshman year was when they switched from the Feynman Lectures to Goodstein teaching from his new The Mechanical Universe. (Goodstein was one of the pair with the negative quote above.) I thought at the time that the new textbook wasn't bad, but that Feynman was amazing. IIRC much of why they wanted a new book and video series was to take advantage of 3d computer graphics; I don't remember that making much of a difference to me.
(Added: I also attended a Feynman lecture live, but can't fairly rate him vs. others because I'm hard of hearing and rarely get much from lectures. The lecture hall was indeed packed with non-freshmen, but it didn't seem like any of us frosh skipped it. This isn't very relevant since it was a one-off.)
The page addresses two points: Feynman's fluctuating opinion of his own work, and the drop-off in student attendance (Feynman's opinion is then regurgitated as the third and final point). In fact, Feynman's opinion has no bearing on whether or not LoP is an empirically effective textbook. And regarding student attendance, the page argues against one person's recollection of dwindling attendance with another person's recollection (years after the fact) of steady attendance. Again, no evidence, just two contradictory and possibly flawed memories that have no bearing on whether or not LoP is effective.
The page provides as evidence an undated photograph of Feynman in front of a full classroom. When was this photo taken? Was it on the first day of class, or in the middle of the semester? Was it during a regular class, or a special lecture? We don't know.
Perhaps the LoP approach really is an effective textbook for teaching undergrads (despite Caltech abandoning it, no other nobody else to my knowledge successfully adopting its methodology), but the cited page doesn't provide relevant facts regarding the matter.