What form do you expect the 'evidence' to take? Aside from politicians making the requests and the companies responding to the requests and the Establishment boasting about it online?
I imagine crushing competitors is another part.
As Glenn mentions in the article, Parler was intended as a platform that wouldn't do the kind of data collection and perception shaping that services like Twitter and Facebook monetize.
If you're not going to shut down that competitor the best thing would be to convince everyone that only Nazis want to use a social media platform that doesn't attempt to track as many aspects of your life as technically possible.
Aside from the balance of political donations and their willingness to de-platform viewpoints that Democrats don't like, what evidence are you looking for? Do you think the tech CEOs signed a letter in blood and video taped a ceremony where they pledge fealty to their Democratic Overlords?
Seriously, what are you looking for to be more convinced?
Glenn is quite liberal, but he loathes authoritarianism (which comes in all flavors) and hypocrisy.
Also, to reciprocate with some effort. Evidence that I would find more compelling:
Testimony/quotes from the involved parties that describe the arrangement/understanding. I'm thinking of management/high level employees at Big Tech, then similar level of people among the Democrat machine. Documents that describe the arrangement/understanding.
If that can't be done, perhaps quotes/perspective from uninvolved parties but whom have been in a similar position at Big Tech or in Government who could explain how the sausage is made. Who would be involved in decision like this, what would be their thought process, etc.
The type of things that are common in investigative reporting as opposed to emotional rants.
If that type of evidence can't be obtained, some level of analysis that shows why every other plausible explanation doesn't have merit might help.
Why can't it be because Parler really is reprehensible place for violence and something reputable companies no longer wish to be associated with? Why is it that holding the Senate for the Dems is what did Parler in rather than holding the House? Etc.
If the other plausible theories can be debunked, perhaps then, the only remaining plausible explanation is that Big Tech is policing right wing discourse because the Dems won the senate.
Those are the types of things I would find more compelling than an observational tweet from someone I've never heard of.