When it comes to Organization XIII in general, they all underestimate Sora quite a bit. Which is strange, because Sora's defeated two halves of Xehanort already (not solo, but still). Then there's Castle Oblivion and everything that happens there.
Xigbar's the only one that understands how much of a threat Sora is, but even in DDD, he was still downplaying him. I think it's once Sora did his "my friends are my power" speech, he started taking Sora more seriously. That's why in KH3, he's less of a troll and more cryptic than usual. Also remarking on how Sora's "rained on their parade more than once."
So I think Xigbar in KH2 is only saying that because up to that point, he hadn't really seen Sora's strength first hand. That, and obviously because he's trying to get a rise out of Sora (Sora knew this and didn't take the bait, though).