It's not a "gotcha", it's correcting the GPs deceptive contrast of labor groups versus white supremacists, when they were one and the same.
> look at how the FBI treated [white supremacist] labor groups versus fascists ones in the 20s [..] no need to worry about the decades after, they killed enough [white supremacist?] leaders to stop [white supremacist??] leftwing voices in the US
So the fact that the labor groups the FBI targeted were white supremacist by today's standards makes those sentences complete nonsense.