Yes it does, by definition. Optional means not integral.
Only difference between game and DLC is you have to pay extra money to get it. That doesn't lessen the importance or how integral it is to the over all story line. It just means you have to cough up more money for it. In fact DLC is even more important then you are trying to make it out to be as DLC gives developers chances to expand on content in game they might not have had a lot of time to do during the original game set up.
Now there is a part of me that understands were you can develop this basic concept. There is a ton of DLC out there that are pointless and have no purpose besides getting a few extra bucks out of the player's wallet. But when you have something that actually develops or expands the game over all enriching it from it's existence then it is no longer optional. Just like to the party. Leviathan and Javik are not optional. New skins for the characters to have is optional and thus not integral.
Unless you can some how prove they had all the DLC pre made before the game was launched and they simply cut out the parts that were not needed so they could simply resell them. Then you might have a point.
DLC should be about expanding the universe or adding extra stuff. DA2's Mark of the Assassin and Legacy were both awesome at this. Lair of the Shadow Broker, Bring Down the Sky, and Overlord were good examples from Mass Effect. They didn't affect the plot, but expanded on the characters or lore. Optional DLC should not be required to understand the plot.
You don't need DLC to understand the plot. It simply expands on the lore of the plot a lot more. The only people who would 100% need the DLC to understand plot are idiots or people who skipped every cut scene and never tried to interact with characters just jumped from one gun fight to another.