While I agree about not having hard and fast rules, like LoC per PR, the principles of this are very relevant.

When reviewing a conglomerate commit in a PR, I have to reverse engineer how the different changes interact to figure out the intent. I then have to do this on each update they make. Contrast that to when someone breaks up their commits where I can zoom through variable renames, extracting functions, etc to see the one line that change that all of that unblocked that makes the difference. Then if updates are pushed, I only have to worry about the commits that were updated.

As for all commits compiling, that is helpful to review the individual commits.

Both of these (small commits, all compiling) are also great for bisecting. You get pointed to a very small change that you can more easily analyze vs dealing with breakages or having to analyze a large change to find what the problem is.