That is true. As a DO, there are five states that require an osteopathic or allopathic internship (that meets the AOA requirements- ie. family practice, peds, IM, surgery, OB, like the traditional rotating internship for MDs). Five states: PA, FL, MI, WV, OK. If you want to practice in any of these states you must have an AOA approved internship or jump through a lot of hoops. Even if you're an attending for 15 years, rumor has it, that if you've not done an AOA approved internship you won't practice in these five states.
The philosophy behind it is to make DOs in well-rounded and to see the complex relationships between other organ systems as they might relate to your specialty. Apparently a rotating internship was required for all MDs prior to the 1970s.
Politically, a big force for requiring the osteopathic internship is to ensure that osteopathic programs are filled...by requiring and internship you're more likely to stay at that same hospital for residency.
All in all, for DOs doing EM...the current status is that DOs in DO EM residencies are all 4 years long. So, the only way you'd be an EM physician without doing 4 years is get into an allopathic program that isn't in one of the 5 states. (There are some unique exceptions where DOs have not had to do an internship in one of the 5 states- the SDN moderator Quinn, for example).
In regards to a COMLEX score of 500 and trying to get into a allopathic residency, I'd recommend either killing your COMLEX II (above 80th percentile) or taking and doing well on the USMLE.