For a long time I mistakenly thought that daddy long-legs were "technically not spiders", just because people mistakenly call crane flies "daddy long-legs", and crane flies are definitely not spiders.
Discussion
People call cellar spiders daddy long-legs too. But they are different from the daddy long-legs spiders that show up en mass and have rounder bodies. And yes crane flies too get that name. Three species at least with the same nickname.
Those are all nicknames or common names. If you’re trying to be specific, you have to learn some scientific names because many common names are used for multiple unrelated species.
In the case of daddy longlegs, Wikipedia says there are spiders, non-spider arachnids, and those crane flies that are all referred to by that name.