Origins of the kilt, is it Celtic or not?

[Schultz]

The Picts as a culture were wiped from the face of the Earth by the Scoti who were a Celtic tribe. The Picti/Scoti later called "Scots" invented the Kilt.

While I agree with you that the kilt is a Scottish garment and not a "pan-Celtic" one in origin, let's not forget that those Scoti that you are referring to came from Ireland.