With help of sci.math crowd it was demonstrated an example that there exist a (finite) poset, which is separable (in the sense defined in this book), but $latex \star x \subseteq \star y$ does not imply $latex x \sqsubseteq y$ (where $latex \sqsubseteq$ denotes our order) for elements $latex x$, $latex y$ of this poset.

See this short article with a diagram of such a poset.

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