TheDcoder that Composer requirement means it can be installed (but not necessarily tested compatible) with any version >=0.1.0-beta.15 and <0.2.0
See https://getcomposer.org/doc/articles/versions.md#caret-version-range- for an in-depth explanation of all the version/requirements mechanics.
Because not all extensions are affected by breaking changes, the minimal requirements do not need to match with the latest Flarum release. Although it's indeed a good indicator for more complex extensions which will most certainly be affected by breaking changes.