That is because you are looking at things in terms of some kind of artificial cuttoff between intermediate and advanced.
He said that intermediates and on should spend as much time at 85% and above. With as much time as possible ABOVE 90%. He didn't mean 85 to 90. This is for intermediate and beyond. Your needs and how you are cycling things at any particular time will determine the kind of intensity you work at.
No one will end up doing all their work at 100% some day
The idea is to accumulate as much work at these very high intensities as you can manage and recover from. You find ways and tricks of doing that. As you get more advanced what changes is what you can and can't tolerate not the relative intensity you need to aim for. An advanced guy may have days in the 85% range and days in the above 90% range just like an intermediate.
So to some up he simply meant that there comes a point that for strength you have to concentrate on intensities of 85% at least and beyond...with emphasis on beyond when possible. He pointed out that 5x5 is simply of too low an intensity for continual gains in absolute strength. You also have to realize that he is writing to a general population and he probably believes, correctly so, that most will never quite reached a REALLY advanced stage.
The how and the why are two different things of course.
You know I was saying how people say intensity is important BUT...
Another big problem is that there are so many airy fairy and personal defintions of "intensity". What I am talking about and what EC is talking about is intensity as a percnetage of you 1RM. Pure and simple. Be careful because many people add concepts to that to come up with some convuluted way of viewing intensity and that is what leads to some of the volume thinking, and the 'advanced' methods like drop sets and shit like that. I'm not just making that up. I just read a thing where someone was talking about drop sets to "add intensity"!
Because they think that intensity is related to how hard it feels and your state of fatigue. So the first thing you need to know when people use terms is how do they define them. When you are looking at strength training parameters and their defintions there really shouldn't be too many unclear distinctions. If your definition of intensity allows that lifting a little pink dumbell 300 times is "intense" because it "feels intense" toward the end...you probalby need a better definition of intensity! One that doesn't allow for such muddy waters.