This is not true... Your AC Aeroplan Pionts will be able to be used anywhere on the star alliance network. The upside is that AC Elite is granted instantly with 35,000 miles. or 50 segments. The down side is that Aeroplan Fees (when flying AC) are more expensive than you would be accustomed to for reward travel on AA or UA.
You can fly AC and credit your miles to UA M+ if you want - but you won't get upgrades flying on AC Metal that way.

Just some thoughts from someone who made Elite this year for the first time and have enjoyed the benefits already on flights within North America.