Right, and I'm not saying it's absolutely definitive, or even my own conception of this distinction. However, if someone personally defines it that way for themselves, I think it's valid.
The key attributes of forums that I think make them superior:
Chronological posts. There is no upranking or downranking. And certainly no voting. Sometimes just an option to "thank" users for posts. I hate the social consensus formation effect of up/downvoting so much that I've disabled visibility of up/downvotes from my lemmy account.
Slower pace. Threads often live much longer, with participants dropping in and out over the course of weeks or months. Sometimes years. Posts themselves are often more thoughtful and better drafted because of this slow pace.
Index structure. Topics are sorted first by their category or subcategory. Exploring into these is like thumbing through a file cabinet. In contrast, the "reddit way" groups topics by community association, more clique-enabling IMO.
Forums often work without any hard javascript requirement.
Also forums are usually one subject that is then broken down into more specific subjects. Car forums are often specific to a model, then the subs would be pictures, cleaning, engines, troubleshooting etc.
Also there's no algorithm trying to push you to click on something.