The punchline of the joke is that the Jews were treated the worst, and the antisemitic party members are going to resent them no matter what.
That's a fair reading, I guess.
That's the only reading.
They have it better because they got to leave first. Party members had to wait in line for hours. That was my reading of it
The punch line is that the anti-Semitic bigotry had the accidental effect of benefiting the Jews, who didn't have to stand in the pointless line as long.
Today, the joke would be accompanied with the "congratulations, you played yourself" meme.