After doing some digging around, I can confirm ACV has both Tango and Flex fares (filed) to the Caribbean. However, what days these fare classes available vary and is due to how many seats (in which fare class) the ACV buyer buys.

The new award chart (for after Nov 28/13) not only aligns rouge to mainline in terms of point accumulation but also ACV. Meaning you earn a % (instead of a block of points like intial Rouge) on ACV as well. Most commonly this will be Tango (50% AQM) and 1 AQS.

The only flex fares I could find (through the ACV website which is a separate booking engine from AC) is to the Bahamas, and Mexico (CUN).

This would explain why AC is allowing upgrades on Tango fares for upper elites, since flex class will not often be available (depending on what ACV buys).

Note ACV flights are just consolidated seats that ACV bought on regular mainline/rouge flights. So if you really wanted Flex class, you could just purchase the same flight on AC.com. But most people who book on ACV, are looking for an all inclusive package and to not book their flights/hotels separate.

So it looks like AC is throwing a bone to the upper elites for booking through ACV, instead of booking their vacation package through a competitor (Sunwing, Transat etc). Especially since Transat (and others) offer a premium cabin of sorts.