As noted in the article there was a Harvard professor who said the same thing but about white people. It was a nuanced position about how groups like unions have acted along racial lines by excluding black people to advance their interests. The underlying point was about how race underlies so much in our society and white people need to stop acting like that.
For reference here's a bit on what the professor meant even though it's not the original direct source, just the professor writing about it.
https://www.harvardmagazine.com/sites/default/files/pdf/2002/09-pdfs/0902-30.pdf
In theory the post could have been a satire of that or just standard run of the mill legal anti-Semitic tropes about how Jewish people are loyal to their religion or Israel above being American. Gross either way since Jewish people aren't equivalent to the social position of non-Jewish white people in terms of historical discrimination.
I don't give the guy credit for it being clever satire, and think it's just anti-Semitism.
The other legal area is the lack of specifics. A threat legally speaking has to be direct, immediate, and actionable. Given he has no realistic ability to carry out his threat, legally it might not meet the threshold.
https://uwm.edu/freespeech/faqs/what-constitutes-a-true-threat/
But one could argue that the specific history of Jewish people would make it a threat in the same way it's illegal to use a cross burning to intimidate black people but I could burn one to protest something else.
That said, my opinion is the guy is at best a racist troll who isn't a serious human much less lawyer in training. At worst he's a potentially violent racist who thankfully doesn't seem to have much charisma to get like-minded idiots to follow him.
Personally I feel skeevy "defending" the guy on these grounds, but the 1st amendment defends a lot of abhorrent words to ensure that it protects a lot of speech that is good but might be unpopular. Any non-state college could absolutely toss the guy out of school and I'd cheer them for it. But the government is held to a different standard.