"The proof of the pudding is in the eating." -- old saying --

I presume you have already hiked enough to be able to test this theory against the reality of your own feet. You say that you have noticed this phenomenon on some of your longer hiking days, so that would argue that, for you, this advice applies, but only on some of your longer hiking days.

That other theory, that the fit needs to be somewhat snug, but overall comfortable, is right on the money, too. However, if your feet change and your shoes do not, then the fit will not stay the same from the start of the hike to the end of the day, rendering that bit of wisdom less useful.

What you need to do is take what you know about your own hiking habits (how often do you hike 10 or more miles) and decide whether you need to optimize the fit during the first 7 or 8 miles, or optimize fit at the end of a long day, to avoid rubbing which could cause lingering problems for subsequent days.

P.S. My feet do NOT swell or flatten, no matter how far I hike; they are extremely small, narrow and bony, so there's practically nothing there that could swell. The advice you are getting seems to apply more generally to larger feet than mine, and applies with greater force to people with the largest shoe sizes.