It is supposed to be to test demand to see whether mainline can operate next year so that the Rouge aircraft can be allocated elsewhere (they are limited in the number of rouge aircraft by union agreements).
Unless that's an internal staff email in relation to testing demand, pinch of salt comes to mind. Moving to mainline is reducing capacity on the route.
They are getting 6 more B767s and AC are phasing out the final 15 B767's as B789's arrive so most of them will be heading to Rouge.
Testing mainline for a route which lasts 3-4 months is not viable at all, a longer Rouge schedule March-October and Year Round operation should be the aim before moving to high cost mainline.
Think it was also discussed fares didn't change to reflect mainline J cabin.