As a matter of fact, you can't express any serious idea in 140 characters. Twitter really is a tool for mobilizing and organizing mobs of people who are already somewhat familiar with and supportive of your ideas, which is exactly why all sorts of activists and politicians use it for their campaigns.
What made the presidential campaign on twitter look the way it did was not really deficit of censorship, but fighting a political battle on grounds where there is no space for actual exchange of ideas but only for appeals to emotions and prejudices, personal attacks and name calling.
The conclusion is: if you don't want populism to win, maybe you should be careful building such platforms. And twitter has been known to be conducive to ugly flamewars for years, go read about gamergate. Maybe parent thought that founders regret not the lack of control but the fact they built such a thing, but then it seems you are right that they actually don't.
The early US election I remember was Bush v Kerry 2004. If you used those exact same words to describe the campaign back then, I wouldn't have disputed it.
Twitter did not even exist until 2006.
Did the internet enable Donald Trump? Absolutely. It enabled him to go face to face with the likes of Jeb Bush and Hillary Clinton, and beat them at their own dirty game.
Everything is made of atoms.
The sun is an ordinary star.
Infections are caused by germs.
I think that is the real problem, but I'm hesitant to say that's what their argument really is unless there's some actual evidence. At best, it's not backed up properly in the posted article, to the point of (to me) being highly misleading if their problems are with the lack of freedom in Twitter for one.
I think Twitter had a net negative effect on political discussion not because of free speech, but because it rewards bombastic language, point scoring, cheerleading and makes substantial arguments impossible.
I don't think there's many popular forms of social media that actually have nuanced, reasonable political discussions though.
EDIT: the only one I really know of is /r/neutralpolitics.
What I mean is that I think the problem currently is as simple (but complex to fix) as people disagreeing over reality.
When I have coherent political discussions with friends of other leanings, we always see the others standpoint, but it ALWAYS boils down to simple disagreement on cost/values.
Let's take wealth inequality: I don't see the utility in the 0.X% population holding YY% of the wealth (instead of YY/2), and think it hurts the economy by stagnating monetary rotation on the market. My more conservative capitalist friend also doesn't see a particular added utility by it, but disagrees about it having the cost I describe, and distributing wealth that way would then be invasive.
We won't get further than that barring a real world experiment, but whether it's healthcare, schooling, infrastructure, etc, it always after 5 minutes or 2 hours boils down to a disagreement on a premise, core values or simply something unprovable (reasonably).
The problem I see publicly (social media etc) is people ignoring reality/facts/common sense in favor of tribalism. At that point there can be no discussion.
I venture into r/neutralpolitics at times too, but I've never once read something fruitful there. It's either entirely noncommittal, or legitimizes both standpoints on an issue that shouldn't reasonably have two sides (whether that's liberal or conservative).
More often than not the argument basically boils on to this [0], and even when it's not, the tweetstorm necessary for a constructive answer gets lost in a sea of raging tweets.
First of all, the architecture of a service has important influence on the kind of speech that it will foster: for example, the 140 character limit insures that it's almost impossible to get a nuanced argument across. Second, the broadcasting model of twitter often leads to horrible dogpiles.
I am a huge free speech advocate, and I don't like censorship but several key decisions about the architecture of twitter were definitely suboptimal if your goal was building a better town square for the exchange of ideas.