...provided you have GREAT legal representation.
Absent that, the best you can do is apologize for the wrongdoing you have committed and comply with ALL of the claimant's demands.
The demands are to be gone... which it already is, there is nothing to lose in challenging the complaint. Even if concerned about retaliation or further legal action from Casio, all this guy did is essentially glue extra stuff inside a calculator case and publish a video of him doing it on youtube, it's not even particularly interesting from a hacker perspective because nothing about the calculator itself was learned, it's a true duct tape mod.
[EDIT]
> Neutrino informs us that he has already filed a DMCA counternotice with Github to get his project back.
And it's back:
Are you sure? See GitHub's DMCA counter notice policy[1], specifically, item 3:
> Provide your contact information. Include your email address, name, telephone number, and physical address.
In other words, without paying to retain legal counsel, this guy would have to expose his personal identifiable information (surrendering anonymity) to an unethical, ruthless pack of legal professionals to restore the repo and live with the looming threat of all sorts of perfectly legal harassment.
[1] https://help.github.com/en/github/site-policy/guide-to-submi...
The code is not even for the calculator, it's for a "esp8266-12E" which is an arduino-like board. This has more to do with the plastic case than electronics or code:
If you watch the video of the modification to the calculator at the bottom of the article you can see the only thing the calculator board appears to share is a power source - this is essentially a duct tape mod that went to the effort of fitting a separate computer and display inside of the case. If you can find a fork (git links in the comments) you will also notice that it's all arduino code.
Casio simply dislikes the mod because it adds a separate computer to their exam-approved calculator case, turning it into something only suited to formative assessment while being difficult to differentiate from the original. However the repo has pretty much nothing to do with their calculator other than the fact that it was once used on an arduino board that was glued inside of a casio calculator in a youtube video - IANAL but pretty sure DMCA has no ground - their statement certainly seems to be a lie based on the fact that the code is not for the Casio, then again they probably also have no legal grounds against people using their own plastic casio calculator cases for something that's not a casio circuit... so this is quite possibly a willful abuse of DMCA processes on Github to get it illegally removed.
[EDIT]
On closer inspection it should be possible to invalidate the complaint on the spot... the full complaint is included in the article, including a link to the supposed "original copyrighted work":
https://support.casio.com/en/support/download.php?cid=004&pi...
It contains some generic example formulas in a simple looking expression language which I expect runs on their calculators. Not exactly something you could set up a http connection with, and definitely not a "literal copy" of the arduino code in the other repo.