The author of this article is Ben Collins, a reporter for NBC that covers disinformation and extremism, primarily. In the tweet, Collins shares that a Revolver News article titled, "The Battle of the Century: Here’s What Happens if Elon Musk Buys Twitter" lays out a plan to declare-- in the words of the article-- a "war against the Globalist American Empire." Collins goes on to state that the article (published April 1, 2022) was texted to Musk 10 days before his acquisition of the platform (April 14, 2022) by an "unknown user" and that this may explain many of Musk's decisions with the platform.
Collin's Twitter video was released on the same date as an article he published, "What was Elon Musk’s strategy for Twitter?." This article dives into the assumption that the "unknown user" who sent Musk the article was Darren Beattie (former Trump White House speechwriter) and that Musk has been following the article as a playbook.
I would say that the claim that Musk bought Twitter to "declare a war against the globalist American empire" is largely an extrapolated line from the Revolver News article. While Collins makes the case that Musk is using this article referenced as a playbook to make decisions for Twitter, the actual decision to buy Twitter and whatever Musk decides to do with it is not a direct call for fire. Additionally, the original video tweet by Collins never states that Musk bought Twitter to "declare war against the globalist American empire," merely that this article may have influenced Musk's decision in acquiring Twitter.