Sora never defeated LS. For one, that battle is arguably non-canon. Secondly, even if it was canon, nobody won the fight. The fight just stopped. Third... Sora had backup with him while LS was on his own.
1. Donald and Goofy were not a big help, they were just alive for what, 5 seconds
2. Nomura majorly said that the fight between Sora and Lingering Sentiment was canon like Aqua vs mistery guy, th new bbs boss
3. and Sora had to win the fight or els Terra would not stop fighting him, he realised that he were not Xehanort, thats the first reason to fight agains him, after so much years, since BBS (Terra's last boss is Xehanort) to KHII (where LS thought that Sora were Xehanort), it looks like LS just have one goal in mind, defeating Xehanort
i wouldn't surprise myself if we had to fight agains LS again, but in an easy battle