You're overly focusing on the term and not the meaning. The term comes about from people choosing tools like "foxit" or "Opera" and saying that those products are safer than their cohorts Adobe/ Firefox because they are attacked less often.
This notion was termed "security through obscurity" ie: "you use the less popular option, therefor that option is safer". It has nothing to do with "obscuring" in the sense of "hiding", that's a linguistic quirk of a colloquial term. If you were actually taking action to reduce the ability to understand a system in a way that you could meaningfully defend, it would no longer be "security through obscurity".
The argument has persisted because there are two different questions that sound the same (X is less typical than Y):
1. Is "X" safer than "Y"?
2. Is a user of "X" safer than a user of "Y"?
When looking at (1) in isolation, you can say things like "X lacks security features, therefor Y is safer" and "X is less often used, therefor X is safer", etc. This is a question about the posture of the project itself, in isolation.
(2) is about the context for users. The reality is that X, which perhaps is fundamentally less well built software, may actually have users who are attacked far less frequently.
Both are likely to favor "rarity is a poor indicator of safety" as we generally reject mitigation approaches that rely on attackers to behave specific ways, but what's important is that these are completely different questions and neither has to do with being obscured but rather rare.
None of this is about what is "obscured" or not. If something is obscured or obfuscated, that is a technique that can be evaluated separately by its own merits (ie: how hard is deobfuscation, how easy is it to adapt to deobfuscation, etc). All of this is about whether you're evaluating (1) or (2) - and in the case of (1), which is what the criticism always has focused on, the answer is that "rarity" is not a mitigation.
