Last time it was only left wing extremists...
Disclaimer: I'm in the worst spot, since I'm right in the centre, so both leftists and rightists can hate on me all day
The problem is, that is not Twitter(the company)'s goal. For the last several years, their main focus, through user complaints, has been to limit the unfettered flow of information, to produce curated feeds, etc. This is quickly converging with Facebook functionality. A recent major development (from outside Twitter itself) has been the introduction of blocklists, directly contradicting the author's utopia of "incorrect tweet followed by a correction". Meanwhile, Twitter has been introducing similar features for "celebrity" accounts.
Given that the article's stated reason for saving Twitter does not align with the company's goals, the only way to save it would be to get rid of the company currently managing it. However, I don't see any other company actually looking to maintain the model described in the article. News sites are shutting down comments sections, removing the possibility of corrections. College campuses, normally a bastion of open debate and conversation, are looking more and more into punishing "hateful" speech and creating "safe spaces". Social media is heavily invested into curation.
The article states "Twitter must be saved". By who? Even Twitter itself does not want Twitter to remain the way the article writer envisions it, and the other major actors are tripping over themselves to go faster than in the opposite direction.
There are in fact places dedicated to disseminating information with as little filter as possible, unlike Twitter. Off the top of my head, I can think of liveleak and 4chan. Neither of them is extremely popular, and 4chan has an awful reputation. I'm surprised that the author is not extending their support to them, instead of trying to support a company that is actively trying to run away from his vision.
https://about.twitter.com/safety/council
After a cursory glance (it's a long list) I don't think any of the organisations strike me as particularly left wing, certainly not in the traditional economic sense.
Twitter also very selectively enforces rules about what it judges as hate speech and abuse. Certain people can repeatedly tweet "kill all police", or advocate jihad, or advocate violence towards those who support a particular political candidate, but the moment anyone does even a fraction of the same towards any group that Twitter is in bed with, that person gets shadow banned or permabanned. Note: I'm not advocating hate speech, merely pointing out that Twitter allows it more often than not when it aligns with their politics.
Twitter had a chance to be a platform for open discussion, but its brownshirt employees chose to make it a censorship shithole. Never-turned-a-profit Twitter can go fuck itself – it's not essential and highly filtered.