As strong as Leicester looked, the final score was only 3-1. This clearly does not mean a fantastic advantage or something like that, and as you correctly noted, Legia had chances to score additional goals, and if they took advantage of these chances, the match could go in a completely different scenario. Therefore, these odds (18) still look like complete insanity.
Only three goals? This is not a bad result at all. Legia is not bad in this season of the Europa League, I would not say that it is a weak opponent, so the outcome of the match as a whole is very good. But if we talk in the context of the odds, yes, it's a strange match and it's not clear why such high bets were made.