Disclosure: I worked on the original MN9 Kickstarter campaign.
FWIW, it's not Day One DLC (the full update spells this out really clearly a couple places, though if you just saw the OT I can see how you might think this). And it's basically saying that all pre-order money (slacker backers) from this point through the end of the year will go towards making this DLC, if they get enough to make it -- and if they do, the 80K backers who already funded the main game will get it for free.
They are showing their costs (which are over what they are saying they need in pre-order money) and seemingly being totally open about the entire process -- I'm not sure how this can be construed as unethical just because the game isn't out yet, unless you think taking money for pre-orders is unethical.
I do know some people personally don't like hearing about DLC before a game is out yet, for any reason, which I can certainly understand. The update makes it sound like (and I know from working on other games) they are probably talking about it now cause of how some parts of game dev finish before others.
I think this is a fair response from the perspective of "I don't think there is anything they're doing wrong business-wise because it all seems legal and it is transparently explained". But I would like to take a moment to put another perspective on this. One which is not about not liking DLC or wanting to hear about it before a game it out. And I'm wondering what your thoughts on it are.
One of the main reasons for crowdfunding to exists is to help people who have ideas but lack funding to get support from like-minded people who want to support that idea and make it happen. It's an alternate funding method which relies on goodwill. Beyond providing money to make the project happen, it also takes a huge amount of risk off the hands of the creators behind a project. I think everyone can agree with this assessment.
Now having said that, lets look at what they're actually offering to make, and what they're asking for in return. Aside from various enhancements and additions put into a game which was fully funded including all stretch goals, they introduced the idea of voice acting. They explained the cost and got more people to support the project, they met the goal and now there will be voices implemented into the game for everyone regardless of when they buy it. I think that's mostly fine. People must have wanted voice acting since they were willing to pay for it, and they got it.
But now we're looking at what looks to be a pay DLC component being added to the project. The reason why paid DLC exists is because content is not cheap to make, and publishers need to be able to make the money from development back to justify the investment in making it. That's why they're paid DLC instead of free bonus content. But in this case, that doesn't apply. They're asking fans to fund the DLC by further supporting the crowdfunding process. They want the DLC to be paid for so it is risk-free for them, and everyone who supports the game gets it for free, sure, but they also want to be able to sell the DLC as its own product. Now for those who didn't support the project, when the game is released, they can sell MN9 to them and they can sell MN9 DLC separately. This is completely a business move, not a "because we love fans" move.
If the DLC is already funded by supporters, why not give it free to everyone who buys the game when it is eventually released? Shovel Knight is going to have a bunch of post-release DLC which they're making now, and that will all be free for anyone who bought Shovel Knight, regardless of being a backer or not, since it was all paid for by supporters.
That's why I think this is a crappy deal and extremely selfish on Inafune's part. He is using goodwill from supporters who are passionate about the game, but he's still operating like a producer at a corporate company. I think it is pretty lame.