A British couple had a run in with a gator at the same lagoon a few weeks ago. They said they told someone. Also the news said that Disney tries to monitor for gators.
So Disney does know gators are possibility. News said signs are posted not to swim, but no signs warning about gators.
But they are wild animals. During droughts or times when their food supply may otherwise diminish, they will go after anything they believe is, not to be insensitive, a meal.
I am not defending Disney or accusing them, but they'll probably be held liable for this either way, at least civilly, and will end up making changes to alter their policies.
That being said, it is almost impossible to prevent wandering, foraging alligators from slipping unseen in and out of the lakes in Florida, including Disney World.