This is patently false.
Many chapters discuss what levels are appropriate, and some even spend a sentence or two on what to do if the characters aren't quite there yet. Example:
Chapter 1: "Prisoners of the Drow" assumes the characters start at 1st level, and that they will achieve 2nd level if not 3rd by the end of this chapter of the adventure."
Chapter 7: "By the time they reach this point, the characters should be at least 7th level, having overcome challenges in their wild and dangeorus journey through the locations decribed in chapters 1 through 6."
But it is a
sandbox. Meaning you're given a whole bunch of encounters, but no set path to play them in.
Blingdenstone? Or Gracklstugh? Could be reached at level 3. Or level 6. Or not at all. Same with
every other descibed location. This doesn't mean the party can bypass
all the adventure content, only that there is nothing described in chapters 3 through 6 that's
essential to further play.
This means that one party might rush ahead and only be level N when a certain chapter is reached, while another party might fan out and explore everything (and by pure happenstance leave this particular chapter for last) and thus be level N+3.
But this is only a problem
if you as the DM do not adapt. This is a
sandbox. It is
your job to monitor the PCs advance and insert certain key encounters at appropriate times.
That chapter 7 advice I quoted above? I read it this way:
"You should strive to have the characters reach this point only when they are at least 7th level"
or
"Insert this chapter once the adventurers reach level 7"
See the difference?
If you expect to just go from page 23 to 24 to 25 in order, boy, you're in for a massive disappointment. This is an adventure for a DM who likes juggling half a dozen alternatives at the same time.
So, really, that party rushing ahead? It won't reach foes that stomp on them because they're only level N.
Not unless you make it so. Your job is to seamlessly insert enough of the material given so this only happens if you want it to happen.
The module tells you to do this in various places. But it expects experienced DMs, and so it doesn't spend much time detailing
how. This is a
sandbox. What it does is throw plenty of random tables at you, and a couple of "set pieces" as well. What you do with these is, however, entirely up to you. As far as the adventure goes, the only criteria is "have at it until they're level seven".
In this particular case, the key to "but how do I keep the party just aimlessly wandering when all they want to do is escape to the surface?"
is discussed,
but not until chapter 7. So you can't just postpone reading it until the party "gets there". Because they won't ever "get there" unless you make it so. And you need to read up on this chapter and in particular the discussion on "Finding the Way" already as soon as the party is about to escape the Drow outpost.
Prep well and you shall be rewarded.
Try to run it like a rollercoaster and... well, I simply suggest you don't. Why?
Because this is a
sandbox.