"Optimal?" Maybe, maybe not. Consider the following:

I'm substantially older than most college students; as a matter of fact, I have a lot more time to train since I retired, unlike the average college student (and, in all likelihood, study participants!). And my conditioning, alas, is likely much lower than the study participants'. So if I posit "optimal" means increasing plasma volume, mitochondria, lactate threshold, muscle glycogen storage, etc. over calendar time (say, by then start of next summer), what's "optimal?" Should I spend 6 months riding 2-4 hours daily in zone 2, and adding in intervals and threshold training after maybe 4 months of zone 2 riding? Or would two half-hour interval sessions weekly be better for getting into shape for next summer?

I don't know whether to cut the study authors some slack, or not. On the one hand, it's a lot easier (and cheaper) to find some college wanna-be-athletes and drive them for half a semester, than it is to do a longer, slower study with more participants, and try to do a decent job of data analysis to pull information out of all the noise such a study would generate. On the other, my impression is that the lit search was rather slap-dash -- like what a junior professor would have some "Introduction to Science and Analysis" students do in a one semester class.