Opinions can absolutely be backed up with evidence, to suggest otherwise is just disingenuous.

That's disingenuous selective quoting, I never said nor suggested otherwise. I was very clear about why I don't need to provide evidence, I said

We're philosophizing, not science'ing; that requires arguments, not evidence.

I did not say opinions can't be backed up with evidence.

you agree that psychology, medicine, etc are valid and have credibility?

As a form of engineering to help people, sure; that's vastly different than using them to base policy and law from. Surely you can see the difference between the DSM being used to help people with problems vs the DSM being used to set policy in government and law for citizens?