The rule that you're talking about is for NHL players joining the AHL. In their case they have to be on the AHL roster on the day of the NHL trade deadline.
You'll see a bunch of teams making paper transactions at the deadline to make age and contract eligible players available for the AHL teams should the parent NHL franchise be eliminated.
There is no such rule (from what I can find) that applies to major junior players joining the AHL when their CHL season is over. Based on the transfer agreement they seem to be perfectly eligible to play.
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