The issue with unity with communists is not merely disagreement between Marxian and Austrian schools of economic thought. It's a disagreement in philosophy: the assertion that "property is theft". Differing moralities on this scale cannot peacefully coexist. Communists would have the perfect moral lease to conquer their neighbors and institute a private-property-less Marxian system, perhaps against the will of their neighbors, simply because they do not believe you have any right to own property, that owning property is aggression, and that they have the moral right to seize your property and return it to the public. This is completely incompatible with actual Rothbardian "propertarian" libertarianism. Unity is only good for them, good for them to take us over.

Libertarians are not against only the state, but against all initiatory violence and coercion, including that by non-state actors. If the act of defending property is considered initiatory violence rather than retaliatory violence, that ideology is fundamentally at odds with everything the Libertarian Party currently stands for, and has stood for since the beginning.