'Friends have in common all things they possess.' This is, undoubtedly, an unselfish high-minded and excellent sentiment. Who could suspect that an argument starting from such a commendable assumption would arrive at a wholly anti-humanitarian conclusion?There are also various hints that Popper likes altruism before and after this. But this is worse than just advocating altruism. It is an unlimited form of altruism where nothing is held back. It tells us that all possessions should be common.
The sentiment also sounds something like a generic attack on people having differences, and therefore a very intolerant statement, but perhaps it's different with more context.
OSE p100: Popper says the term 'individualism' has two dictionary meanings. The first is the opposite of collectivism. The second is the opposite of altruism. He says one of Plato's tactics was to lump both senses of individualism together, in order to argue for collectivism by attacking selfishness (an invalid and dishonest approach). And Popper separates them out, in part for accuracy, and in part so he can defend anti-collectivism without having to defend selfishness, and can say the first sense of individualism is compatible with altruism.
I think Popper's dictionary is correct to connect these two concepts. If you value individuals in their own right, then how can you advocate those individuals all individually choose to sacrifice themselves for others? That is not the standard individualist attitude of people caring deeply about their own lives, and taking responsibility for themselves, and pursuing their own interests. Altruism is a way of sneaking collectivism in through the back door.
Popper only goes half way in his support of individualism. He opposes collectivism but not altruism. Ayn Rand goes the whole way. She vigorously supports both meanings of individualism. I wonder did anyone else before her ever seriously defend selfishness? I mean for a good reason, not something like advocating tyranny and trying to justify powerful people selfishly keeping their power.