What I think we see sometimes on the forum is that some riders are looking to train year round, outside of work / family and want to get the best value out of their limited training time, and, in particular, raise their FTP.
Some guys also want to train for a long time with one goal in mind, their first race, an A race etc.
Some of us (myself included) are looking to prepare for an entire season, and have plenty of time to train "old school" with a long base, followed up by a period of intensity to fine tune FTP etc.
What I think the author is saying is that there can be risks in "riding as hard as you can" each time you train because you will quickly become "permanently fatigued" and your performance will stagnate. That isn't the same thing as saying "if you train L3 a lot you will not get any better". I don't think he is also advocating old school base training especially vociferously. He is just warning against riding "too fast too often" especially in the off season.
FWIW the last 2 seasons I rode Cat 1 and only rode crits. Thus in the winter I did (mainly indoor) more intense workouts of shorter durations, pretty much only L3, 4, 5 and 6. I was terrified of losing my speed, and thought that unless I was suffering I was stagnating.
At the start of the season yes I was fast and fit, and good enough to finish races. However, as the season went on I noticed I was not really improving at all, and my legs were constantly fatigued. When I was riding with elite riders I noticed I could match them for a short duration, but would find myself gasping to maintain what they lay down for longer periods seemingly effortlessly. To try and correct this I went out and trained even harder, more intervals, more L5, and then ended up doing 3 day blocks of 90 minute "hard as I can" rides, then 2 days resy before a crit. Sure, I got good at being in the hurt box for 90 mins, but generally I got slower, tired and demotivated. The reasoning process was "I got slower, I need to train faster" etc etc It got unpleasant.
This season I have moved up to elite, and since November I have been laying down 4-5 hr endurance rides on the weekends, and 3 x 2-3hr "sweet spot" based indoor sessions in the week. (240-280w / 62 - 75% of Max HR, Highest possible cadence 95+) With commuting as well I have been 20hrs a week for a month or so now, but with hardly anything over low to mid L4.
Right now I feel really strong, as if I am charging a battery - I have this feeling of freshness in my legs, and I really feel that by having clocked up so much duration in the lower intensity aerobic / sweet spot zones when it comes to turning on the speed I will quickly see some impressive results. I'll also not be fatigued from months of agony through the winter.
When season starts I'll be able to use the (weekly) classics in the spring to get massive durations in L3/4/5 which will then give me a great base to build the top end of the engine in the crits.