For example, people who live in South America usually have a lot worse internet connection, and more likely to be on mobile networks, than people in the west. If you're audience is mainly in South America, working on making your application accessible, means making sure your application has low bandwidth usage, can deal with high latency and jitter, and works well for phones.
One only has to look at the definition of the word "Accessibility" to see why you're wrong: "accessibility - the quality of being able to be reached or entered - the quality of being easy to obtain or use - the quality of being easily understood or appreciated".
Not sure where the focus on people with disabilities came from, it's important, but accessibility is much bigger than just that.
You are right, accessibility _can_ take into account degraded environments, but if you don't know why disabilities are focused on, that suggests to me you are philosophizing on the word, and not addressing actual technical standards that have been put in place. Just google it
Your argument was "Accessibility is specifically about making things accessible to those with disabilities" which by no definition of the word, is true. Look it up.
Not sure why I'd have to read the various specifications again when I've already done that. I'm still of the same impression (like most others) that accessibility is much bigger than just making things work for those with disabilities.
But if it's more accessible depends on the target user. If you're target are IE users or users with very old browser versions, for one reason or another, more accessible would be to use the CSS toggle trick. But if indeed your target user are users using screenreaders and using recent browsers, then details and summary elements would be more accessible.
A bit anal, but important to consider when talking about accessibility, it's very context sensitive.