The games has a problem far bigger than lack of resources: Lack of vision. You can feel the budget, but you can see it ill-used. This DLC didn't drain resources from the main game, but chances are it will share its problems. Fun Fact: If it's indeed 5 hours it's going to be longer than every 2D entry at speedrun.
Long does not equal good.
But from what I've heard of the game, your assessment seems accurate. The game looks like it has a severe case of ADHD; all of these random ideas and plot points were thought of, and it seems like they bit off more than they could chew trying to cram all of it into the game.
And DLC is a bit of a grey area, because unless we're talking about a developer with some amazing corporate transparency, consumers just aren't going to know what had to be cut due to lack of resources/time or what was just planned out for DLC and squirreled away.
I don't care much one way or the other, because in the end I pretty much always ignore DLC anyway. Most cases of DLC, however, give me the impression that a developer thinks their game is worth more than the initial $60 and doesn't want to come out and say so, instead putting more content behind a pay wall and marking it as an "extra".
It's impossible to deny that this looks suspicious when a game like LoS2 is rushed out the door, seemingly incomplete, and then suddenly the DLC is announced.
All this speculation likely comes off as masked caterwauling, especially when someone could just as easily wait for the game's price to drop a third or two. But this sort of "business first" approach to games really, REALLY makes it suck for day-1ers who really look forward to a new release.
EDIT: I just remembered that I kind of conflated "developer" and "publisher". I wouldn't be surprised if the final decision for putting out DLC came from Konami themselves and not MercurySteam.