a person who I'm fairly certain was not a Jew themselves, actually declared me not Jewish because I live in the U.S. Which is just another form of antisemitism.
Antisemitism (or, more accurately, 'Judeophobia') is being bigoted against someone because they are Jewish. What that person said to you was not antisemitism, it was just stupid.
Overusing and misusing (sometimes intentionally- à la Israel) that accusation dilutes it's severity and makes people more likely to handwave actual incidents of discrimination and bigotry, which is not good my Squid.