Where did you get this belief/perception?
It was harder to get into cards, and GI - but definitely not insanely hard. Having gone through the process myself (and later on, personally involved in selecting and interviewing candidates) - your residency plays a much bigger role than medical school, along with board scores, LORs (who wrote it, was it glowing or luke warm), as well as research (posters at regional or national, peer-reviewed publications, etc)
Being a DO at a small community hospital AOA program would make it harder to match Cards/GI/HemeOnc/PCCM ... than if that same DO were at a university-affiliated hospital, or a university hospital. Part of it is name recognition, part of it is resources (faculty that does research, is known within the field, knows faculty at fellowship programs, etc) ... I've seen the same bias against MDs as well who are from small community hospital (US IMG, FMG, and US MDs) ... I think there is a stronger bias against small community programs than against DOs
Now is this universal for every program? No. There may be a few staunchly anti-DO program, but for the most part ... the further away from med school you are, the less important it becomes
Now is the field completely level - NO. You will still have trouble trying to get Cleveland Clinic Cards, UCSF Pulm/CCM, MD Anderson Heme/Onc - but not sure if it's just DO bias, or the competition (MDs attending top IM programs with very impressive CV/resumes)