A lot of people seem to conflate ‘democracy’ with ‘status quo institutions and center-left parties’, but in many cases these are deeply illiberal and undemocratic. I think you would benefit from considering institutional / center-left threats to democracy, which quite glossed over in this essay.
When I think about threats to democracy, I think of things like:
Cancelling or repeatedly delaying elections if your party is expected to lose.
Preventing opposition legislators from being allowed to vote.
None of these violations of individual rights or the ability of the people to affect policy through political change were enacted by populist or extremist parties—they were enacted by generally respected and left-wing incumbents.
The article described Thiel attempting to persuade people to leave the pledge, but not that he has actually succeeded.