AC requires that you be at the departure gate no later than T-15. Thus, if you were on the 10:15 (AC 606), you needed to be at the gate no later than 10:00. It does not matter whether the flight had closed when you arrived at the gate.

So, the very specific question to OP is at what time did he arrive at the xYYZ departure gate?

The auto-rebooking is solely a convenience and to assure that OP has an onward flight. He is free to have rejected the onwards reroute and to have cancelled and asked to be return to his origin.