[The Saucepan Man]

OK, so its not definitive, but this is from the entry for Silvan Elves in The Illustrated Tolien Encyclopedia:

Those Elves who undertook the Great Journey were the Teleri. Of this kindred were those called the Nandor, "those who turn back", who stopped their westward march at the Anduin River and went no further. Of these Nandor, there were some who settled inGreenwood, and Lothlorien. These were named Silvan Elves, for most of them lived in forests.

So, the name "Silvan" seems to denote their habitat, ie forests, rather than their descent. This is consistent with the dictionary entry for "Silvan" or "Sylvan": "of the woods; having woods; wooded", from the woodland deity, Sylvanus.

So rather than describing a particular race of Elves, doesn't "Sylvan" just mean those who live in the forests of Greenwood and Lothlorien. In which case, Legolas and Thranduil are Silvan Elves, notwithstanding their Sindari descent.