It's not. It's maybe even worse than racist, because most of Disraeli's ancestors probably weren't even members Solomon's kingdom, they likely were mostly Europeans who converted and then adopted the ancestry.
Because the person who says that is part of the oppressed minority, not the other way around.
Why is that relevant to if the statement is racist? Yes there is a difference between kicking someone when they're down and kicking while down, but it is still kicking.
Disraeli said that in the quarrel as a response to O'Connell who've referred to Disraeli's Jewish ancestry in disparaging terms.