The rule is indeed to use the past perfect continuous (for actions that were in progress) and past perfect (for completed actions) after “it was his opinion” This is a type of reported speech, so we need to back-shift. Could you just use past continuous and simple past? Yeah, that would be acceptable in conversation but it would be sloppy in formal situations or in writing. Or on an exam, if that’s your interest. In some situations, it would even be unacceptably ambiguous (I picture your dialog in a professional police investigation or even a courtroom setting).
On the other hand, facts that are always true do not necessarily require back-shifting: “the teacher taught that the Earth is round.” Also, if the report was recent and the facts haven’t changed, we don’t have to back-shift: “I just told you that I like pizza.”