You know what? It's great advice for some writers, with some stories. But like all one-size-fits-all advice, it actually doesn't necessarily fit most very well.
Me, for example. My first drafts tend to grow by 10-20% on redraft, because I tend to write my first drafts without things like transitions, exposition, dialogue, dramatization, and setup for thematic developments. They're more or less nothing except plot and character development, and all the other stuff gets put in later. I also have to insert white space a lot of the time, because at novel length extremely dense narrative becomes exhausting, and that's what I naturally tend towards.
My earliest decent short stories were all around 1500-3000 words. It wasn't until I learned to unpack those, to get the interesting bits out of my head and trust that I wasn't going to make them boring by explaining them, to write them at 5000-7000 words for the same sorts of ideas, that they started selling well and attracting positive comment.
I've several times shown my students the first draft of "Shoggoths in Bloom" as well as the published version. The story won a Hugo in its longer, published version. The first draft was largely opaque and hasty, and I know this for a fact, because I took it to Sycamore Hill and sat in a room while a dozen of my colleagues told me exactly how much of it was incomprehensible twaddle.
(Not always: sometimes I need to cut things. But it's incredibly rare for that to happen, and feel free to ask my editors.)
In my experience as a teacher of writing--going on ten years of it now--this is true for a good third of my students, as well. Some of whom have struggled extensively because of this advice, which gets parroted around as if it were true for everyone, all the time.
It's not. Just as the advice to "expand that--dramatize that--explain that better" is not true for everyone all the time.
(In point of fact, I suspect that there are no generally applicable answers even for particular writers. Sometimes we'll overwrite, and sometimes we'll underwrite, and experience and good editors will eventually teach us which is which.)
The trick is not to apply some magic metric like "Oh, cut 10% of everything you write." The trick is to learn what information is necessary and what information is not, and provide the former--and as much of the latter as is entertaining and fun.