High-quality corporate training hinges on maximizing student engagement. This typically means having students spend time in class writing programs. (In one four day class, I've had students write >20 programs.)
IDLE is a surprisingly good tool for this.
It's cross-platform and in the standard library. It postpones questions about which is the best editor/IDE. It avoids the tab/spaces problems. In fact, it's just about the simplest tool for teaching interactive development (i.e., coding with access to a REPL.) (I should note that this style of programming is distinct from how quants and data scientists work: they write more analyses than programs/scripts. When doing corporate training for quants and data scientists, Jupyter Notebook is the only way to go.)
By the end of the class, most students are pretty tired of using IDLE. It's not a very good editor; only barely better than Notepad. As the article's screenshots show, it just looks ugly. It's not even close in functionality to an IDE or a `vim`/`emacs`+plugins. The REPL is extremely clumsy to use in practice (especially compared to `bpython` or `ipython`.) In fact, I would be shocked if anyone actually uses IDLE as part of their regular job.
That said, it's a great teaching tool, and I'm glad to see someone putting effort into improving it.