You know, considering my relative was good friends with Reinsdorf due to their involvement AEPi, I feel like I can speak with authority on this.
No, it's not.
Now if people were saying he was being "miserly", "Jewy, etc., or used certain imagery, yeah, that's anti-Semitic. Saying he's cheap due to not wanting to spend big on free agents, draft picks, etc. is not.
He has spent on the Sox in recent years (especially on amateur talent), so the mantra is weak and may be rooted in fan ignorance and lack of wanting to do research about spending habits (likely) or rooted in anti-Semitism (unlikely, because I'm gonna guess most fans don't know he's Jewish, because most people aren't that attentive).
So, don't use our people as a shield because we're not allowing you to speculate about the personal life about a young man during a difficult point in his life.
This where my piece ends on the matter.