Yeah the CHL offers a bit more flexibility when signing an ELC with an NHL team. If you're in the CHL, you can get the security of the ELC and still go back to juniors if you're not ready for the NHL. But yes, if you're still 19 before January 1st of the current season, it's either the CHL or NHL. You cannot play in the AHL.
If you're in college, once you sign your ELC, you cannot go back to college. But you're eligible for the AHL no matter how old you are. So Artyom Levshunov, who did go pro out of college immediately after being drafted, is an exception of someone who was drafted out of college and went straight to the AHL. If he played in the CHL last year, he would have had to stay in juniors this season or go to the NHL.
It's also a decision for a college 18 or 19 year old to make as they are unlikely to be ready for the NHL at that age. So it's a choice of living the college life or riding busses between Rockford and Des Moines in the minors.