Activision is definitely forcing Bungie to hold stuff back, but it's unrelated to the CoD release. Activision owns both IPs, so it would be pointless to have them compete with each other. Destiny is not meant to attract the same audience as the people who play CoD, it's going for a different demographic. Specifically, they want to attract people who enjoy Skyrim and Borderlands while making money off the poor people who thought this was going to be Halo reincarnated. Since Skyrim and Borderlands both had extremely successful DLC campaigns, Activision has planned the same for Bungie, except rather than producing new content after the launch of the game they have deliberately cut content from the initial release rather than extend the release deadline of Destiny.

I had that thought too