The point is to get work done, not do it all in a single window. you talked about "capability" as if because someone uses an external debugger you can't debug effectively or very well when using something like vim.
It's a flawed definition of effective.
When you use something like vim, the entire ecosystem is your IDE.
If the entire ecosystem is your IDE, you're not "Turning Sublime Text into a Lightweight Python IDE".
I don't know how else to phrase this to make it clear that I'm not attacking, denigrating, claiming inferior, etc., composite workflows like vim + GDB. But if you get to an article that says XYZ is now your IDE, and it offers a piss-poor debugging experience unless you leave XYZ, then that article is not turning XYZ into an IDE.