[William Cloud Hicklin]

Whatever T had in mind when he wrote the Lorien chapter, around the time of Pearl Harbor, and whatever he might have speculated in very late scribblings, if we need a 'canonical' answer then I think we are forced to accept Appendix B, which was published and therefore carries the authorial stamp of 'finality': Celeborn was a kinsman of Thingol, who did not live in Lorien in the Second Age, and thus was of Doriathrin origin. And this itself counts as "late", since it was added to the Second Edition in 1965; the First Ed read "...many of the Sindar passed eastward and established realms in the forests far away. The chief of these were Thranduil in the north of Greenwood the Great, and Celeborn in the south of the forest. But the wife of Celeborn was Noldorin: Galadriel sister of Felagund of the House of Finrod."

So both ca 1955, and formally ratified a decade later, Celeborn was a Sinda. (Note that as of the First Edition, Oropher didn't yet exist)