^that would make sense. The Ivy League Agreement (https://admissions.dartmouth.edu/apply/fine-print/common-ivy-league-agreement) requires that all Ivies release RD results on one day simultaneously. No other schools to my knowledge have such codified arrangements with other schools. In looking up the agreement, I see that LLs are actually discussed explicitly (I had assumed they were a sort of a backdoor loophole around the shared results date).
Looks like @T26E4 is right in so far as the LL is no more rescindable than a formal acceptance offer.
Interestingly, the agreement specifies that only LLs from the admissions office “count” as acceptances. A coach can talk up an athlete’s chances all he/she wants to the applicant and unlike the formal LL, it can be arbitrarily rescinded for any reason, even if given in writing: