If screaming “apartheid” has an impact in delegitimising the very basis of Israel’s existence, accusing it of genocide is a still more powerful weapon. So it has, likewise, been repeated until it has now become accepted as fact – such that it can be asserted without contradiction.
Neither apartheid nor genocide have been proven, but as we've seen with far too many things like especially with climate science denial repeating a lie over and over can make people believe anything sadly.
