In the UK you could sue for libel, because this suit potentially defames the public character of the accused and could cause both personal and professional losses.

It would put the burden of proof on Casio, requiring convincing proof that the claims in the lawsuit are true and neither vexatious nor frivolous.

That would be interesting to watch, because I get the impression Casio's legal team have no idea what they're doing. They're really trying to protect Casio's status in exams, but to do that they're talking about source code and using the DMCA as a legal gambit.

I doubt they're really interested in whether source code was used or even what source code is. Someone on the team thought "OK - sounds good - we can make that stick" and here we are.

If it's really not true - which sounds likely, from the comments - it's sloppy and lazy work and leaves any number of open goals for countersuits.

In the US you only can't sue for libel because DMCA preempts all federal and state causes of action - instead you sue for misrepresentation under 512(f).

But for false claims of infringement that don't involve copyright, a libel claim can work. I've previously mentioned my own 8 figure lawsuit against tp-link for false claims of counterfeiting; I have antitrust, tortious interference, declaratory judgement, and defamation (i.e. libel) causes of action. I know of at least one similar case with a multi-million dollar jury award from 1999 (see Alexander binzel v nu-tecsys.)