I'm aware that third-party Chinese and Japanese IMEs are a thing, but what user-facing difference does the third-party IME you mention provide in the Hangul context?
- Support for less-popular or customized keyboard layouts. From time to time popular OSes had a wrong version of supported layouts as well.
- Deterministic autocorrection. For example, normally an initial jamo (e.g. ㄱ) should be followed by a medial jamo (e.g. ㅏ) and not vice versa, but many IMEs offer a feature that automatically swaps them. With a carefully designed layout this can catch lots of transposition typos.
- Custom candidates for special characters, as Korean IMEs show special characters as candidates when a jamo is being "converted" to Hanja (popularized by MS IME).
The IME in question [1] also allows an extremely customizable input system, to the extent that it forms a soft of wholesale DSL for Hangul IMEs.
[1] And when the affected user base is small enough, even larger problem can take a lot to fix. I use a third-party IME that is relatively known among programmers, and early this year Google Chrome began to crash seemingly at random when you type Hangul. I tracked the root cause and I believe it is ultimately Chrome's fault, but I was too busy to file an issue at that time and the IME has adapted to Chrome to avoid triggering the crash next month. I believe Chrome still crashes when older versions of the IME are in use.