I guess everybody has to decide where to put the boundary between engine and game. If you're making a conscious decision that the game developer doesn't have to write code in your development language, that's fine. But you have to provide a robust framework in the development language. Depending on what you want the game developer to be able to do, you usually have to also provide a programming language (or more than one) so that they can design complicated interactions.
Different game frameworks/engines provide for game developers to create individual content in different ways. Crawl has its "vault" language, Angband has a bunch of different datafile formats, TOME has LuaScript, and so on. To me, the crucial thing to recognize is that game content is, or at least can be, complicated enough to require programming, so if you *have* content that complicated, the game developer has to have access to a programming language, and has to write code in it. It can be an language more suited to the problems you're defining, or have restricted complexity/capabilities, or any of a bunch of things to make the game content easier to develop. But there's a certain level of complexity you just can't pass without using a programming language or something equivalent. Programming languages, after all, are just tools for managing procedural complexity.
In the software industry we see tools, every so often, that say you can develop complete applications without doing any programming. They are invariably wrong. They're either unusable for anything outside a very narrow class of programs, or they're just reframing the complexity-management in a different programming language while pretending that the "configuration files" or "webpages with extensions" or "Database table layout" or whatever, despite equivalent complexity management, aren't actually programming. I really wish new middle managers would quit getting sucked in by those, because they make my life as a programmer actually harder.
Bear