I don't think he's saying that at all. I think the point was to elucidate that many undergrad classes are more intellectually demanding than med school and that success in med school is primarily a work ethic/ efficiency hurdle with everyone meeting the requisite intellectual barrier. I tend to agree with that, at least for the preclinical years. Organic, calc physics, and upper level molec bio classes were orders of magnitude more intellectually difficult than what I've seen MS1-MS2 and qbanks. I'm sure that varies by undergrad and major. With that said, the volume and detail ends up making med school way more difficult.
For example here's a med school question: description of patient presentation, some background history, some lab reports, and histological description of a kidney biopsy. Sifting through the vignette requires intelligence and some students will confuse lets say renal cell carcinoma, ARPKD, and ADPKD. However, the majority of students will agree the vignette is describing a cystic disease. Looking at an associated image confirms ARPKD in your mind. However, the answer choices are all listed as chromosome numbers, demanding students to know the location of the mutation. The vast majority of students will reason to this point, know that it's the PKD gene, but only 20% of students will have spent the time memorizing the chromosome number even though they have the intellectual capacity to completely understand renal pathology.
Of course there will be a few more difficult questions on each exam, but those are creating spread at the top, not separating a near pass and an honors grade. The OP was asking about people really struggling to get by and I think there are a bunch of other factors that come into play before you make it down to intelligence differences between students in the same class.