Actually, it doesn't say who the foot belonged to. However, it uses the word 'great' to describe the foot, and just a few lines before Gandalf uses the word 'great' to describe what he believes was a cave-troll. He merely uses 'large' for the Uruks. So I believe it is implied that Frodo did in fact stab the foot of the troll.
We should always remember that Treebeard doesn't know everything that he thinks he knows, to paraphrase Tolkien. He might be spouting gibberish about Trolls. Who knows? We cannot take Treebeard's words as truth, simply by virtue of their being Treebeard's words.
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Los Ingobernables de Harlond
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