It's a new global variable. It isn't the AddOnManager which you can get by calling GetAddOnManager(), it's just the UI object.
Pre 1.6.3:
Lua Code:
function ZO_MainMenuManager:Initialize(control)
local addOnManager = ZO_AddOnManager:New(true)
1.6.3:
Lua Code:
function ZO_MainMenuManager:Initialize(control)
ADD_ON_MANAGER = ZO_AddOnManager:New(true)