You're assuming they'd put it in though. In the past, if they had a new awesome idea, either they'd put it in the main game or they'd just be like "we don't have the budget, so we can't do it" and not put it in at all. Now stuff that might have fallen in to that category they can release as launch DLC, charge separate for it, and so be able to include it.
But they are developing it, they clearly have the budget to make that idea a reality. Does it require the developer to say "yeah we're totally cashing on on this" to convince you? No dev's going to admit to that. They've got the ideas during development, they have the funding to make it a reality, they have the framework to add it right into the game. However rather than doing they, they're selling it as a separate component.
They have the budget to make it if they charge extra for it. That's all we really know for sure, whether or not they would have the budget without charging more for it is a guessing game.
And no, they don't have to come straight out and say it, I just need some evidence beyond the fact that it's release-day. One of those things could be that it's content already on the disc, and the "DLC" is really just an unlock code. Another would be if the game feels significantly incomplete without it. Or if they advertise like it's built into the game, but then it's released separately. There are lots of things that could go into that.
Question: What if it's release-date DLC, but it wasn't developed concurrently. Like the dev made the game, sent the final code to Q/A and to be pressed/shipped to retailers/whatever. Then, after that was done, they started working on DLC, but because the DLC is so much smaller and they already have the engine and game framework and stuff set up they're able to get the DLC done before the game actually releases and have it up for download day-one. Is that okay?