The action desribed in that tweet would be illegal, but is the tweet itself illegal? Why should the tweet be deleted? I'd rather keep it for posterity.
If I happen to mention to you that it would be a shame if my associates and I were to decide to burn your nice little business down in the night, but maybe if you pay us that won't happen I would assume that you would know that a threat was being made. The arson is illegal, but so is the threat to commit it unless you do something that is to my material benefit.
It's very often illegal to threaten illegal retribution. Employees may not know their rights, and such a threat could discourage them from exercising those rights.
Deleting the tweet is maybe not ideal. Any future viewers of it should have the context added that it's an illegal threat.
I would say what happens to the tweet itself is almost an afterthought—it was one of only many violations addressed by the report, along with more mundane stuff, like contract provisions about employees speaking to the press (ruling: it's unlawful to prevent employees from speaking with the press as long as they make it clear they don't represent the company). The NLRB also required them to post notices addressing the tweet at all factories, and it was immortalized in a 55 page public document—not to mention all of the contemporaneous news reports. I think it's very safe to say that posterity is not going to have a hard time finding this tweet
