It is possible he had not heard of Karl Marx, given that he had just moved to the US. He might have heard of communism and the Soviet Union, though.
Also, yes there was no reason for him to fear living in Uganda at that time. His book is about the Ugandan-Asians struggling with identity and belonging, and they were sort of stateless at that time. So when Idi Amin expelled them it was something of a gift to them for it helped force Britain to accept them as British citizens.
