Without being registered officially it cannot technically be referred to as a cultivar, so no, it is not. A coined name for a plant someone picked out, perhaps, but as someone else could more than easily take the name and apply it to another plant and register it officially, it has no validity.I'm wondering if the person selling the "Silwia" on eBay is actually just misspelling "Sylwia." It's still a cultivar, just not a registered cultivar.
Without being registered officially it cannot technically be referred to as a cultivar, so no, it is not. A coined name for a plant someone picked out, perhaps, but as someone else could more than easily take the name and apply it to another plant and register it officially, it has no validity.
CULTIVAR noun. Botany. A plant variety that has been produced in cultivation by selective breeding.
There are a number of definitions for "cultivar" and I think the most accurate is "A variety of a plant that has been created or selected intentionally and maintained through cultivation" which encompasses both naturally occurring cultivars like Sarracenia leucophylla 'Hurricane Creek White' as well as hybrids.
Perhaps I should have made my wording more along the lines of: unless registered, the cultivar does not have an official name, just a marker that can still be used by anyone for any plant. It's still a "cultivated variety," but nothing to protect its recognition.