1) He's telling people to use Window's built in defragmenter, which Windows is already doing, not a third party program.
2) I can't find any sources saying Windows only defrags smaller files. Even if this were true, defragging does not help fps. Defragging only consolidates information on your hard drive. This means it's only going to effect load times and how "snappy" your system feels. Deleting your downloads folder like he said is just not helpful in any way when it comes to FPS. If he was claiming load times, or general system feel, I'd agree. Unfortunately, these don't have a measurable impact on minecraft's in game performance and telling people that it does is incredibly misleading. More so, he refuses to read comments, so at this point I'm 90% sure that he's just looking to post farm by making easy guides and "looking" helpful.
Otherwise though I totally agree with you! Running a more efficient defragging program could help a ton of people with older systems feel better running programs and just using their pc in general. While getting rid of chrome's insane clutter won't help your minecraft, it will do wonders for your system's responsiveness.