I think you're overestimating how much effort it really took for HBS to create a user-friendly editor. For one, it doesn't take too much time from anyone but the programmers... and more importantly, it was done as part of the process of creating the devs' own toolchain, which they had to do anyway. You'd have a point if you were talking about a game (or series) that already had an established engine with the appropriate dev-only tools, but that's not the case here. They have explicitly stated in the past that they're using the editor themselves to create all of the game's content.
Sure, if you're not releasing the editor, you can have annoying bugs in it that all the writers are aware of (and work around them), but otherwise, if you design the toolchain from the ground up, there shouldn't be a huge overhead at all. If they knew from day 0 that they weren't going to release their own internal editor, how would've that improved the game? It could've shaved off a few weeks of programmer/QA time (and possibly cost some extra content developer time due to a worse-quality editor), and that's it.