Cynically, the purpose of the expert witness is someone to provide something which is treated as fact but isn't subject to penalties of perjury.
Not sure how it legally works, I think in the legal system at least in the US it is quite common that experts are hired and giving opinions based on who hires them?
And then the other party can cross examine? Not enough knowledge on that but it’s interesting if legal people could chime in on how it works.
I hope those expert “opinions” are not taken as facts?