While every communist is a socialist, not every socialist is a communist. c -> s is true, but not the reverse. Thus logically excluding interchangeability

Logically if someone is a socialist but not a communist than most likely they're not much of a socialist to begin with (like being a doctor who wants to treat the symptoms but not the disease), at that point the differentiation is meaningless and very likely an example of anti-communist angst, Marx and Engels had it right in the beginning, there's no point in concealing our aims or glorifying half measures

Lenin's stage theory was very useful in a world dominated by Imperial European empires with no opposition or concept of restraint, so much of Lenin's writing was constrained by his awareness Tsarist censors could clap him at anytime

Now conditions have changed and a century of liberal opportunists and appropriation has made the differentiation toxic and useless to socialist/communist movements