> It leaves um, uh, er and elongated versions (ummmm, uhhhhh) alone. Those sound like fillers but they’re doing real work in the sentence, and cutting them automatically would change what someone said. The rule erm follows: only remove things that are sound, not language.
> It also doesn’t touch repeated words, false starts, or long thinking pauses. Those aren’t noise on top of the speech; they are the speech, just messier than the speaker would like. Cleaning them up is an editorial decision about which take to keep, and erm doesn’t have an opinion about that.
Think about it. Cleaning these things-that-can-be-just-sounds-but-can-also-very-much-be-load-bearing up is an editorial decision. At the very least, you need to judge based on the surrounding content whether the removal of an um would change the meaning at all; and I don’t think text alone is adequate for that.