It's not a "purity" point of view, it's an "ease of use for the purpose" point of view. JavaScript does weird things that stymie experienced devs - a newbie hitting them will be completely lost. What's worse, as I said, is that it does these things because it's poorly designed/its design doesn't adequately cover all the edge cases it should, so newbies will learn the wrong information about why things do or don't happen.
The edge cases are not easily avoided because you don't know what a beginner will or won't try to do when working on their own. That means that not only might they do an okay thing in most languages that doesn't work in JS, but that they may also do something BAD that DOES work in JS, because JS is weird. For instance, the way JS handles variable hoisting and scoping is bonkers compared to most other languages - not stuff you can avoid while learning to program, and making it easy to learn the wrong practice as a result.
"80% of everything is crap" is a good thing to know when doing development work, but it isn't helpful when learning. The goal of learning in this context is mastery of the basics of development; 80% won't be good enough.
