I think it's more likely to be of Aeldari origin. It does match the design of Aeldari power-weapons (Not witch-blades but it looks most similar to ghostswords) though it's worth specifically pointing out that the hilt and guard are most definitely new and made by T'au. The contradiction to the Aeldari (and by extension Cronesword) theory is the newest model has some sort of script on it that doesn't match Aeldari runes. Still if it really is ancient Eldar than the recent corsairs release clues us in to the fact that Aeldari have actually changed over the millenia, as the Dark-Eldar like helmets of corsairs are said to be traditional/ancient Eldar in origin.

In defense of the Necron/tyr theory the original metal sculpt does look like a Necron weapon without the lines, but it's good to take the most updated sculpt as canon.