You can look at it from the perspective that they're only a few months apart in age, which suggests it's very close, or from the perspective one played a year earlier in junior, which points out that that the earlier experience likely helps.
Now, none of that talks about ability, but when the CHL needs to be a proper development league and can only do so by staying profitable to an extent - especially with their education scholarships - having to give 4 years of eligibility isn't such a big deal. If all the stars left when they got close then the CHL as a whole would make less money as a lesser product, and that'd mean less money to develop players so we wouldn't have the likes of Horvat and Shinkaruk as far along as they are.