How does one react to something like that?!

And at least in the US- if you refuse to be sworn in you will likely be warned, and then they will attempt instead of “swearing you in,” to get an affirmation that you understand whatever you say is under penalty of perjury. If you remain elusive to this- the most likely course is that you will not be allowed to testify- which if you are the one on trial or have a berated interest in the success of the proceedings would not be wise. One can always exercise their rights (in America) to refuse to answer questions which could incriminate them or implicate them of another crime. It’s a funny joke though in the anscence of fact- it is a funny idea to picture the scenario.