What is too obvious to explain in a scientific paper?
It all depends on your audience.
In The Structure of Scientific Revolutions, p. 20, Kuhn laments how scientists pick up where textbooks leave off and specialize their writing into "brief articles addressed only to professional colleagues," those "whose knowledge of a shared paradigm can be assumed, and who prove to be the only ones able to read the papers addressed to them."
In general, it is better to broaden one's audience by simple explanations and definitions that do not individually require significant expansion of the paper length (including by citation to longer explanations that would significantly lengthen things), than to limit oneself to a smaller audience whose actual knowledge matches your assumptions.
Here, you're getting clear feedback from the editor/reviewers about things that need to be clarified, so definitely do those things. The editor knows your audience better than you do, and the request is quite reasonable (even much more light than requests for changes often are).
Symbols should almost always be defined, and acronyms should be spelled out on first use, even if well known. This can be done within a sentence (e.g. ", where c is the speed of light in a vacuum") or parenthetical (e.g. "the New York Stock Exchange (NYSE)").
For other terms with specific meanings, it is helpful to define those specific meanings, to make it clear in your paper and help readers distinguish your specific meaning from all the other misuses of the term they've heard. Where specific definitions vary (and they do, a lot more often than you'd expect), it's helpful to cite a source for that definition just as you'd cite a source for any other fact you've included from the literature.