It's not true. Socialism can go well (and is going well) with many sorts of other *isms, simply by re-definition, or making compromises for "greater good". Soviet Union had some semi-hidden policies which broke its own pledges, but it also had a lot of official manifestations, which apparently weren't seen as uncomfortable. E.g. Soviet anthem mentioned "great Russia" in the first stanza. Official Kremlin guard was selected on a combination of ideological, and racial features. And, believe me, I can continue with this for a long time.