Editorializing is twitter reading the tweet, deciding whether it is true or false, and then showing their decision to the end user together with the tweet.
It is somewhat similar to choosing a title for a article, even if you leave the content unchanged you can significantly change the message or the effect by choosing an appropriate title.
In this context twitter is not simply recommending accounts you might like, or refusing to deliver tweets, they are actually engaging with the speech of the user.
As an analogy consider someone that retweets and article discussing some crimes. this person post a link and says: "I can't believe Person A is actually a murderer", in this case regardless of the article content the poster is exposed to a potential lawsuit in case Person A can prove its innocence.
Twitter is closer to this example that to simply choosing to remove content.
(specifically this does not apply to the removed trump tweets)
Nope, that is simply not what editorializing means, in any sense of the word.
Twitter stating a categorically true fact or linking to a resource that debunks factually incorrect statements is not editorializing because they are not giving an opinion. They are adding context with verifiable truths. Which, again, not editorializing because stating a truth is not an opinion.
I'm sure you can dream up all sorts of hypotheticals that makes this sound problematic, such as implying twitter are the ones deciding something is true or false. They don't need to decide that, they just need to recognise a statement as false and then add clarification. If Trump's tweets had any kind of ambiguity then maybe what you say has credence, but that isn't the reality of the situation.
I my example so did the user that shared an article on a murder.
Now, I believe twitter was careful enough not to do anything too stupid, and we all can imagine that Trump's tweet might have been quite extreme. But fact checking in-place is editorializing.
For taking down harmful things, they are just as liable under 230 as they would have been before 230, namely not liable.
Consider how judges around the country have responded to his campaign's legal filings, it's not like Twitter was out on a limb here.