it wouldn't change Disney's liability. They absolutely have a duty of care to keep guests safe.
The question is simply... is warning your customers of the specific threat required to satisfy that duty of care? That is being argued in this thread that because the threat is previously known, regardless of people's education when it comes to the topic of risk of wildlife, Disney is negligent for failing to warn guests of this danger of gators being present in waterways in Florida.
A position that does not depend on the person's prior knowledge, that does not depend on the specific timing or presence of an immediate threat that differs from every other normal day.... but predicated on the idea the threat is known, credible, and can result in actual harm. Relying on the idea that someone was attacked 30 years ago certainly isn't bounding the issue by a specific, localized impending threat.
Just like the threat of someone looking to inflict mass casualties is an ongoing, well known threat, and Disney not only is a theoretical target, but has been confirmed to be a specific target in the past... and such attacks have happened at other public facilities.
Here's a pretty good article on the duty of care as it relates to terrorism (including legal citations). It applies to the sports industry (stadiums), but it would absolutely apply to theme parks.
http://www.martindale.com/legal-library/Article_Abstract.aspx?an=entertainment-sports&id=2342
This speaks mainly to the topic of applicability of the duty of care to the stadium scenario vs terrorists... but barely speaks at all on opinion on what actions would constitute meeting that duty of care. Specifically to this question, when it comes to disclosure or warning... not really addressed.
The general idea of 'acts of terror' being enough for excluding liability has pretty much been long torn up. Cases have long found ways to argue the location/business didn't do enough to prevent it, etc. I intentionally avoided the word 'terrorism' before because of that worry of exclusion... but it has been weakened long ago.
I'm more interested in applying this mantra of duty to warn people of long standing, persistent, risks that would not be unreasonable to known.