As far as I can tell, the fact that the Chicago EA program is open makes a substantial difference. That makes it possible for someone to apply ED at Dartmouth for example, and also apply EA at Chicago. That student would have to go to Dartmouth if they got in there, but could apply to Chicago simultaneously. Some of the ED schools allow applying to another early program as long as it’s non-binding…
Someone couldn’t apply ED to Dartmouth (again as one example) and also EA to HYP or Stanford though, as those schools have single choice/restrictive EA programs.
I think the net result is that Chicago gets a lot of EA applications and also admissions knows the program is open. So I wouldn’t expect applying EA to Chicago to carry the same advantage that it does at some other schools (especially ED ones where admissions knows they can increase yield by admitting ED applicants).