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.