I don't think they are claiming it IS the flag. Notice the key words: "possibility" and "could be" in the first sentence and "may be" in the second. The rest of the article has definitive statements.

Research indicates there is a possibility that this unidentified red palmetto flag could be the Spirit Flag of the Citadel. ... This flag may be the flag that Citadel troops flew over Morris Island as they repelled the Star of the West on Jan. 9, 1861.

Ed